Here are the classes, structs, unions and interfaces with brief descriptions:

Implementor
Gdk::Color	Gdk::Color is used to describe an allocated or unallocated color
Gdk::Cursor	This represents a cursor
Gdk::Device	A Gdk::Device instance contains a detailed description of an extended input device
Gdk::DeviceManager	Functions for handling input devices
Gdk::Display	Gdk::Display object's purpose is two fold: To grab/ungrab keyboard focus and mouse pointer To manage and provide information about the Gdk::Screen(s) available for this Gdk::Display
Gdk::DisplayManager	The purpose of the GdkDisplayManager singleton object is to offer notification when displays appear or disappear or the default display changes
Gdk::DragContext	A Gdk::DragContext holds information about a drag in progress
Gdk::Event
Gdk::Pixbuf
Gdk::PixbufAnimation	The gdk-pixbuf library provides a simple mechanism to load and represent animations
Gdk::PixbufAnimationIter	An iterator which points to a certain position in a PixbufAnimation
Gdk::PixbufError	Exception class for Gdk::Pixbuf errors
Gdk::PixbufFormat
Gdk::PixbufLoader
Gdk::Point	This is a simple structure containing an x and y coordinate of a point
Gdk::Rectangle	Gdk::Rectangle is a structure holding the position and size of a rectangle
Gdk::RGBA	An RGBA Color
Gdk::Screen	Object representing a physical screen Gdk::Screen objects are the GDK representation of a physical screen
Gdk::TimeCoord	A Gdk::TimeCoord instance contains a single event of motion history
Gdk::Visual	A Gdk::Visual describes a particular video hardware display format
Gdk::Window	A Gdk::Window is a rectangular region on the screen
Glib::Value< Glib::RefPtr< Gtk::RecentInfo > >
Gtk::AboutDialog	The AboutDialog offers a simple way to display information about a program like its logo, name, copyright, website and license
Gtk::AccelGroup	A Gtk::AccelGroup represents a group of keyboard accelerators, typically attached to a toplevel Gtk::Window (with Gtk::Window::add_accel_group())
Gtk::AccelKey	Defines accelerator key combinations
Gtk::AccelLabel	A label which displays an accelerator key on the right of the text
Gtk::Action	An action which can be triggered by a menu or toolbar item
Gtk::ActionGroup
Gtk::Activatable	Activatable widgets can be connected to a Gtk::Action and reflect the state of the action
Gtk::Adjustment	A class representing an adjustable bounded value
Gtk::Alignment	A widget which controls the alignment and size of its child
Gtk::AppChooser	This interface can be implemented by widgets which allow the user to choose an application (typically for the purpose of opening a file)
Gtk::AppChooserButton	A button to launch an application chooser dialog
Gtk::AppChooserDialog	An application chooser dialog
Gtk::AppChooserWidget	An application chooser widget that can be embedded in other widgets
Gtk::Arrow	Produces an arrow pointing in one of the four cardinal directions
Gtk::AspectFrame	A frame that constrains its child to a particular aspect ratio
Gtk::Assistant	A widget used to guide users through multi-step operations
Gtk::Bin	A container with just one child
Gtk::Border
Gtk::Box	Organizes child widgets into a rectangular area
Gtk::Buildable	A base class for objects that can be built by Gtk::Builder
Gtk::Builder	Build an interface from a UI definition description
Gtk::BuilderError	Exception class for Gdk::Builder errors
Gtk::BuiltinStockID	See the list of pre-defined stock items, in the Stock namespace
Gtk::Button	A widget that creates a signal when clicked on
Gtk::ButtonBox	A container for arranging buttons
Gtk::Calendar	Display a calendar and/or allow the user to select a date
Gtk::CellArea	An abstract base class for laying out CellRenderers
Gtk::CellAreaBox	A cell area that renders CellRenderers into a row or a colum, depending on its orientation
Gtk::CellAreaContext	This stores geometrical information for a series of rows in a CellArea
Gtk::CellEditable	Interface for widgets which are used for editing cells
Gtk::CellLayout	An interface for packing cells
Gtk::CellRenderer	CellRenderers are used by Gtk::TreeView columns to render the Gtk::TreeModel column data appropriately
Gtk::CellRendererAccel	Renders a keyboard accelerator in a cell
Gtk::CellRendererCombo	Renders a combobox in a cell
Gtk::CellRendererPixbuf	Renders a pixbuf in a cell
Gtk::CellRendererProgress	Renders numbers as progress bars
Gtk::CellRendererSpin	Renders a spin button in a cell
Gtk::CellRendererSpinner	Renders a spinning animation in a cell
Gtk::CellRendererText	Renders text in a cell
Gtk::CellRendererToggle	Renders a toggle button in a cell
Gtk::CellView	A widget displaying a single row of a TreeModel
Gtk::CheckButton	Create widgets with a discrete toggle button
Gtk::CheckMenuItem	A menu item that maintains the state of a boolean value in addition to a Gtk::MenuItem's usual role in activating application code
Gtk::Clipboard	The Clipboard object represents a clipboard of data shared between different processes or between different widgets in the same process
Gtk::ColorButton	A button to launch a color selection dialog
Gtk::ColorSelection	A widget used to select a color
Gtk::ColorSelectionDialog	This dialog allows the user to select a color
Gtk::ComboBox	A widget used to choose from a list of items
Gtk::ComboBoxText	This is a simple variant of ComboBox that hides the model-view complexity for simple text-only use cases
Gtk::Container	Abstract container class
Gtk::CssProvider	CSS-like styling for widgets, implementing the StyleProvider base class
Gtk::Dialog	Create popup windows
Gtk::DrawingArea	A widget for custom user interface elements
Gtk::Editable	Base class for text-editing widgets
Gtk::Entry	A single line text entry field
Gtk::EntryBuffer	A Text buffer for the Entry widget
Gtk::EntryCompletion	Completion functionality for Gtk::Entry
Gtk::EventBox	Event capturing box
Gtk::Expander	A container which can hide its child
Gtk::FileChooser	Gtk::FileChooser is an interface that can be implemented by file selection widgets
Gtk::FileChooserButton	A button to launch a file chooser dialog
Gtk::FileChooserDialog	Convenient file chooser window
Gtk::FileChooserError	Exception class for Gdk::FileChooser errors
Gtk::FileChooserWidget	File chooser widget that can be embedded in other widgets
Gtk::FileFilter	A filter for selecting a file subset
Gtk::FileFilter::Info
Gtk::Fixed	A container which allows you to position widgets at fixed coordinates
Gtk::FontButton	A button to launch a font selection dialog
Gtk::FontChooser	An interface implemented by widgets displaying fonts
Gtk::FontChooserDialog	A dialog box for selecting fonts
Gtk::FontChooserWidget	A dialog box for selecting fonts
Gtk::FontSelection	A widget for selecting fonts
Gtk::FontSelectionDialog	A dialog box for selecting fonts
Gtk::Frame	A Gtk::Bin with a decorative frame and optional label
Gtk::Grid	A container which arranges its child widgets in rows and columns
Gtk::HandleBox	A widget for detachable window portions
Gtk::HBox	Horizontal Box for laying widgets in a horizontal row
Gtk::HButtonBox	A container for arranging buttons horizontally
Gtk::HPaned	The Gtk::HPaned widget is a container widget with two children arranged horizontally
Gtk::HScale	A horizontal slider for selecting values
Gtk::HScrollbar	A horizontal scrollbar
Gtk::HSeparator	Horizontal line widget
Gtk::IconFactory
Gtk::IconInfo
Gtk::IconSet	This manages a set of variants of a particular icon An IconSet contains variants for different sizes and widget states
Gtk::IconSize	Represents registered icon sizes
Gtk::IconSource
Gtk::IconTheme
Gtk::IconThemeError	Exception class for Gdk::IconTheme errors
Gtk::IconView	The IconView provides an alternative view of a list model
Gtk::Image	A widget displaying an image
Gtk::ImageMenuItem	A menu item with an icon
Gtk::InfoBar	This widget that can be used to show messages to the user without showing a dialog
Gtk::Invisible	This widget is used internally in GTK+, and is probably not useful for application developers
Gtk::Label	A widget that displays a small to medium amount of text
Gtk::Layout	Infinite scrollable area containing child widgets and/or custom drawing
Gtk::LinkButton	Create buttons bound to a URL
Gtk::ListStore	Thist is a list model for use with a Gtk::TreeView widget
Gtk::ListViewText	A simple listbox which presents some lines of information in columns and lets the user select some of them
Gtk::ListViewText::TextModelColumns
Gtk::Main	Main application class
Gtk::Menu	A drop-down menu consisting of Gtk::MenuItem objects which can be navigated and activated by the user to perform application functions
Gtk::MenuBar	A standard menu bar which usually holds Gtk::Menu submenu items
Gtk::MenuItem	Child item for menus
Gtk::MenuShell	The abstract base class for Gtk::Menu and Gtk::MenuBar
Gtk::MenuToolButton	A Gtk::ToolItem containing a toggle button
Gtk::MessageDialog	Convenient message window
Gtk::Misc	A base class for widgets with alignments and padding
Gtk::Notebook	Container which shows one of its children at a time, in tabbed windows
Gtk::NumerableIcon	A Gio::Icon that allows numbered emblems
Gtk::Object	Gtk::Object is the base class for all widgets, and for a few non-widget objects such as Gtk::Adjustment
Gtk::OffscreenWindow	A top-level container widget used to manage offscreen rendering of child widgets
Gtk::Orientable	This interface is implemented by all widgets that can be oriented horizontally or vertically
Gtk::PageSetup	A PageSetup object stores the page size, orientation and margins
Gtk::PageSetupUnixDialog	PageSetupUnixDialog implements a page setup dialog for platforms which don't provide a native page setup dialog, like Unix
Gtk::Paned	A widget with two adjustable panes
Gtk::PaperSize	PaperSize handles paper sizes
Gtk::Plug
Gtk::PrintContext	A PrintContext encapsulates context information that is required when drawing pages for printing, such as the cairo context and important parameters like page size and resolution
Gtk::Printer	A Printer object represents a printer
Gtk::PrintError
Gtk::PrintJob	A PrintJob object represents a job that is sent to a printer
Gtk::PrintOperation	PrintOperation is the high-level, portable printing API
Gtk::PrintOperationPreview
Gtk::PrintSettings	A PrintSettings object represents the settings of a print dialog in a system-independent way
Gtk::PrintSettings::Keys	Print settings keys
Gtk::PrintSettings::PageRange
Gtk::PrintUnixDialog	PrintUnixDialog implements a print dialog for platforms which don't provide a native print dialog, like Unix
Gtk::ProgressBar	A widget which indicates progress visually
Gtk::RadioAction	An action of which only one in a group can be active
Gtk::RadioButton	A single radio button performs the same basic function as a Gtk::CheckButton, as its position in the object hierarchy reflects
Gtk::RadioButtonGroup	RadioButtonGroup identifier for RadioButtons To set up this RadioButtonGroup, construct a RadioButtonGroup and then pass it to the constructor of all radio items
Gtk::RadioMenuItem	A CheckMenuItem that belongs to a group
Gtk::RadioToolButton	A toolbar item that contains a radio button
Gtk::Range	Base class for widgets which visualize an adjustment
Gtk::RecentAction	An action of which represents a list of recently used files
Gtk::RecentChooser	RecentChooser is an interface that can be implemented by widgets displaying the list of recently used files
Gtk::RecentChooserDialog	RecentChooserDialog is a dialog box suitable for displaying the recently used documents
Gtk::RecentChooserError	Exception class for Gtk::RecentChooser errors
Gtk::RecentChooserMenu	RecentChooserMenu is a widget suitable for displaying recently used files inside a menu
Gtk::RecentChooserWidget	RecentChooserWidget is a widget suitable for selecting recently used files
Gtk::RecentFilter	RecentFilter can be used to restrict the files being shown in a RecentChooser
Gtk::RecentFilter::Info	A RecentFilter::Info class is used to pass information about the tested file to filter()
Gtk::RecentInfo	Contains information found when looking up an entry of the recently used files list
Gtk::RecentManager	RecentManager provides a facility for adding, removing and looking up recently used files
Gtk::RecentManager::Data	Meta-data passed to add_item()
Gtk::RecentManagerError	Exception class for Gtk::RecentManager errors
Gtk::Scale	A slider widget for selecting a value from a range
Gtk::ScaleButton	A button which pops up a scale widget
Gtk::Scrollable	This interface is implemented by widgets with native scrolling ability
Gtk::Scrollbar	A Scrollbar
Gtk::ScrolledWindow	Adds scrollbars to its child widget
Gtk::SelectionData
Gtk::Separator	A separator widget
Gtk::SeparatorMenuItem	A separator used to group items within a menu
Gtk::SeparatorToolItem	A toolbar item that separates groups of other toolbar items
Gtk::Settings	Sharing settings between applications
Gtk::SizeGroup	Gtk::SizeGroup provides a mechanism for grouping a number of widgets together so they all request the same amount of space
Gtk::Socket	Container for widgets from other processes
Gtk::SpinButton	Numeric Entry with up/down buttons Slightly misnamed, this should be called a SpinEntry
Gtk::Spinner	A widget that displays a spinner animation
Gtk::Statusbar	Text status indicator This widget is used to display status information
Gtk::StatusIcon	The "system tray" or notification area is normally used for transient icons that indicate some special state
Gtk::StockID	See also Gtk::BuiltinStockID
Gtk::StockItem
Gtk::StyleContext	This object stores styling information affecting a widget defined by WidgetPath
Gtk::StyleProvider	This is a base class used to provide style information to a StyleContext
Gtk::Switch	A "light switch"-style toggle
Gtk::Table	Pack widgets in regular patterns
Gtk::TargetEntry
Gtk::TargetList
Gtk::TearoffMenuItem	A special Gtk::MenuItem which is used to tear off and reattach its menu
Gtk::TextAttributes
Gtk::TextBuffer	Multi-line attributed text that can be displayed by one or more Gtk::TextView widgets
Gtk::TextChildAnchor	A TextChildAnchor is a spot in the buffer where child widgets can be "anchored" (inserted inline, as if they were characters)
Gtk::TextIter	Typefed as Gtk::TextBuffer::iterator
Gtk::TextMark	Typedefed as Gtk::TextBuffer::Mark
Gtk::TextTag	Typedefed as Gtk::TextBuffer::Tag
Gtk::TextTagTable	Typedefed as Gtk::TextBuffer::TagTable
Gtk::TextView	Multi-line text editing widget
Gtk::ToggleAction	An action which can be toggled between two states
Gtk::ToggleButton	A Gtk::ToggleButton will remain 'pressed-in' when clicked
Gtk::ToggleToolButton	A Gtk::ToolItem containing a toggle button
Gtk::Toolbar	Bars of buttons and other widgets
Gtk::ToolButton	A Gtk::ToolItem subclass that displays buttons
Gtk::ToolItem
Gtk::ToolItemGroup	A ToolItemGroup is used together with ToolPalette to add ToolItems to a palette-like container with different categories and drag and drop support
Gtk::ToolPalette	A ToolPalette allows you to add ToolItems to a palette-like container with various categories and drag and drop support
Gtk::ToolShell	A base class for containers containing ToolItem widgets
Gtk::Tooltip	Add tips to your widgets
Gtk::TreeDragDest
Gtk::TreeDragSource
Gtk::TreeIter	A Gtk::TreeModel::iterator is a reference to a specific node on a specific model
Gtk::TreeIterBase
Gtk::TreeModel	This class defines a generic tree interface for use by the Gtk::TreeView widget
Gtk::TreeModelColumn< T >	A Gtk::TreeModelColumn describes the C++ type of the data in a model column, and identifies that column in the model
Gtk::TreeModelColumnBase	Base class of TreeModelColumn templates
Gtk::TreeModelColumnRecord	Typedefed as TreeModel::ColumnRecord
Gtk::TreeModelFilter
Gtk::TreeModelSort	A wrapper which makes an underlying Gtk::TreeModel sortable
Gtk::TreeNodeChildren	Typedefed as TreeModel::Children
Gtk::TreePath	A path is essentially a potential node
Gtk::TreeRow	Typedefed as TreeModel::Row
Gtk::TreeRowReference	Typedefed as Gtk::TreeModel::RowReference
Gtk::TreeSelection	Typedefed as Gtk::TreeView::Selection
Gtk::TreeSortable
Gtk::TreeStore
Gtk::TreeValueProxy< ColumnType >
Gtk::TreeView	Displays the model (Gtk::TreeModel) data and allows the user to interact with it
Gtk::TreeViewColumn	Typedefed as Gtk::TreeView::Column
Gtk::UIManager	Constructing menus and toolbars from an XML description
Gtk::VBox	Vertical Box for laying widgets in a vertical row
Gtk::VButtonBox	A container for arranging buttons vertically
Gtk::Viewport	An adapter which makes widgets scrollable
Gtk::VolumeButton	A button which pops up a volume control
Gtk::VPaned	The Gtk::VPaned widget is a container widget with two children arranged vertically
Gtk::VScale	A vertical slider for selecting values
Gtk::VScrollbar	A vertical scrollbar
Gtk::VSeparator	Vertical line widget
Gtk::Widget	Abstract Widget (Base class for all widgets)
Gtk::WidgetPath	This represents a widget hierarchy from the topmost widget, typically a toplevel, to any child
Gtk::Window	Toplevel Window This represents all widgets which are physical windows controlled by the window manager
Gtk::WindowGroup	Limit the effect of grabs