First we are going to look at the structure of the GNOME libraries to see what they can offer. Here is a listing of the different libraries that are present in the gnome-libs package:
Toolkit independent utility library
Toolkit dependent library
Library for using ORBit corba implementation with gnome
xmHTML widget ported to gtk, used in the help browser
A very lean and mean terminal emulator widget
A virtual file-system library used in Midnight Commander
A library used for nice anti-aliased graphics
We will not cover gtk-xmhtml, zvt, libvfs, libart_lgpl and libgnorba as they are mostly specialty libraries.
We can see a clear division between the libgnome and libgnomeui libraries. The former is used in a toolkit independent fashion and could even be used for command line programs that never use X. The latter is the library which supplies the standard widgets and an application framework for applications written using GTK+. It is conceivable to write applications with other toolkits, but nobody as of yet has written a libgnomeui with a different toolkit, and I doubt it will happen soon, as GTK+ is a really great toolkit.