On the GTK+ site, take special note of these pages:
http://www.gtk.org/rdp/ is the GTK+ Reference Documentation Project; look here for reference materials on the GTK+ API.
http://www.gtk.org/faq/ has the GTK+ FAQ; asking these questions on the mailing list is discouraged. Appendix C of this book supplements the official GTK+ FAQ with still more questions; check it out as well.
On the Gnome site, have a look at these pages:
http://www.gnome.org/lxr/ contains browsable, searchable, hypertext copies of all the code on the GTK+/Gnome CVS server. This includes GTK+ and Gnome, as well as many applications.
http://bugs.gnome.org/ allows you to browse GTK+ and Gnome bug reports, and submit new ones. If you find a bug, please submit it here so the maintainers can keep track of it.
http://developer.gnome.org was not yet live at press time, but will be available soon. It will contain comprehensive resources for Gnome developers.