GTK+ and Gnome each have home pages; https://www.gtk.org/
On the GTK+ site, take special note of these pages:
https://www.gtk.org/rdp/ is the GTK+ Reference
Documentation Project; look here for reference
materials on the GTK+ API.
https://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
On the Gnome site, have a look at these pages:
https://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.
https://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.
https://developer.gnome.org was not yet live at
press time, but will be available soon. It will contain
comprehensive resources for Gnome developers.