Except this has been happening for quite a while, hence why a couple of cross platform projects have migrated from Gtk to Qt, including Subsurface, a bit ironically, given the relationship of the project to Linus.
The Subsurface developer did that 10 years ago and it only was because he personally preferred Qt. Take a step back for a moment and consider that in 10 years that's the only major example that anyone ever brings out. GTK is still very welcoming for contributions to maintain the GDK backends. Developers like that have to actually step up and do it and have patience, instead of outright quitting and running off to Qt which has a whole company to maintain those ports.
After seeing this whole thread a day late, I have to wonder: is the unspoken difference what "cross-platform" and "always" mean to different posters? To someone with my historical perspective, it grates a bit to see X Windows conflated with Linux as a platform.
My memory of the early days is consistent with what the wikipedia page says about GIMP. It was cross-platform on the typical Unix workstations that were around the UC Berkeley campus labs and XCF. This was things like Solaris, SunOS, HP-UX, Ultrix, and Irix.
Students in this milieu were just as likely to have some BSD variant on their home PC as Linux. I think it was later during and after the "Beowulf" scientific computing period when Linux started to dominate as the Unix-like platform for open source development.
Even in those days, Gtk+ applications were quite horrible on non-X11 platforms. GTK has never been a good cross-platform toolkit in contrast to e.g. Qt.
Maybe it’s a thing on Windows (I don’t know), but I’ve never seen anyone use GIMP or Inkscape on macOS. I’m pretty sure they exist somewhere, but all Mac users I know use Photoshop, Pixelmator or Affinity Photo rather than GIMP.