This is just a myth. You make it sound like there was some purposeful intention to gimp and gtk. There was not. Gimp was created by some kids at berkeley who created gtk because at the time qt and motif had untenable cost and or licensing, or sucked. When the gnome project started years later gtk was chosen due to licensing considerations since ironically qt was “too free” ie pure gpl which limited its use in closed source software without licensing. At the time this was considered a major problem in unseating windows (back in 1998 when there was actually real vested interest that could be a thing)—- visual studio cost a little bit of money (but it was cheap) and you didn’t have to license anything to make closed source windows apps. You could write a GUI app in assembly and not pay ms a dime if you wanted. The Unix side of things was not so good. Motif was thousands of dollars.

Anyway, turns out maintaining a professional image editing program is a lot of work. While it is true many fanboys named gimp as some kind of foss and gtk showcase its not like most of them were actively contributing to either gtk or gimp.

There was never any longterm large scale commercial investment in gimp similar to how redhat threw massive resources into gnome and gtk.