Gtk became "the default" because Qt had some dual licensing weirdness until qt3/qt4 (I didn't pay that much attention so don't quote me) and by that time severa

Gtk became "the default" because Qt had some dual licensing weirdness until qt3/qt4 (I didn't pay that much attention so don't quote me) and by that time several fundamental projects like Firefox, gimp(of course) and OpenOffice (koffice took until like ~09 to get production ready) had established gtk as the primary Linux toolkit Gnome just follows on because the gtk team is a very purple Venn diagram with the gnome team (at least for paid developers)