The GNOME Menu will be represented as a hierarchical series of directories on the filesystem. Each component of the menu is either another directory (leading to a subfolder), or a text file (which should end with a '.desktop' extension) of the structure:
[Desktop Entry]
Encoding=UTF-8
Name=GNOME Mines
Comment=Mines game
Exec=gnomine
Icon=gnome-gnomine.png
Terminal=false
Type=Application
Categories=GNOME;Application;Game;PuzzleGame;
X-GNOME-Bugzilla-Bugzilla=GNOME
X-GNOME-Bugzilla-Product=gnome-games
X-GNOME-Bugzilla-Component=gnomine
StartupNotify=true
This structure is defined by the freedesktop.org Desktop Entry Specification. These files are stored in $(prefix)/share/applications . Every user has the possibility to add or override entries in the system-wide configuration. The easiest way to edit a menu entry is right-clicking it in the GNOME Menu and selecting Properties from the popup menu. The changes will be stored under ~/.gnome2/vfolders.