GTK is an Acronym for the GimpToolKit.
It is actually called GTK+, but the little plus is almost always left off in practice.
GTK official website: http://www.gtk.org
Check out this Gtk+ Forum hosted by Nabble. It currently archives all the Gtk mailing lists into a single forum to allow cross search and browse. It's a place where you can post questions and find answers and get updated with what the Gtk community is doing.
31 pages link to GTK: