if you look on github, you'll notice that all the obvious names were taken, e.g. go-gtk, go-gtk3. that was what really drove the project name to be gotk3.
gnome tool kit. Note that gnome is pronounced gi'nome just like gnu is pronounced gi'new.
gimp toolkit. gtk is older than gnome and was made by the gimp devs.
I still remember Gimp's first release. Uhm, I guess I'm getting really old.