I tried the Flatpak development release of GIMP 3 and there are already big problems with UI consistency.
For example the forced theming makes it unusable on KDE, this issue describes the too big UI very well and in general it seems like it doesnt make sense to have these huge UI elements on a compact App like GIMP.
Meanwhile apps like Inkscape are on GTK4 afaik, and GTK stands for GIMP ToolKit (or GNU ToolKit?). So its kinda ironic they are upgrading to GTK3 just now.
It seems they are also just doing some cleanup in the code, so I wonder how big the step to GTK4 would be?
And it problems like too large UI elements or theming problems could be fixed.
If I had to guess when GIMP will be native GTK 4 I’d say no sooner than five years, more likely it’ll be 10. By the time GIMP runs on GTK 4 we’ll be working with GTK 5 or 6.
I don’t know how long it will take but it should be much much less work now that gnarly UI elements as old as GTK have been replaced with modern toolkit ones.