The GDK library provides a layer of abstraction that sits between GTK+ widgets and applications and the underlying windowing system. Instead of making calls directly to the X window system, applications call GDK when they need to draw to the screen or handle events.
This extra layer of abstraction provides several advantages. First, it increase portability. Porting GTK+ (and hence, to a large part, GNOME) to another windowing system reduces to porting the GDK layer. A port to Microsoft Windows has already been done. Also, it allows GTK+ programs to transparently use a number of X extensions that may or may not be present. If they are not present, GDK provides substitute functionality in terms of standard X calls. Finally, in many cases, the GDK calls are simpler than the corresponding X calls. Some rarely used parameters are omitted and the correct values for other parameters are determined automatically.