> G(nome)T(ool)K(it). It always was.

I believe it was the "Gimp Tool Kit" originally, and Wikipedia agrees:

> GTK (formerly GIMP ToolKit[3] and GTK+[4]) is a free open-source widget toolkit for creating graphical user interfaces (GUIs)[5]

Maybe the confusion is that it was sometimes called "GTK+" in the brand name originally (though most people just said "GTK" anyway, and that's what the code identifiers said).

Gimp (the photo/image editor) was already pretty sophisticated for years before Gnome was even started.

The first Gnome-branded library that I recall in the GTK space was Gnome Canvas.

The earlier commenter's point stands. Though my impression was that GTK was de facto taken over by Gnome around GTK 3 (though you could still write GUI apps without bringing in the fleet of Gnome desktop services).