GIMP 3.0 has been more than one decade in the making as the port from GTK2 to GTK3, also transitioning away from Python 2 to Python 3 support, and a wealth of other improvements from the UI to lower down into enhancing this open-source Photoshop alternative.
The GIMP project announced on X/Twitter today that they have entered the string freeze for this much anticipated release.
They have been migrating to GTK3 since before GTK4 was out.
I am sure they will stay behind but future porting should be easier.
That said, non-GNOME GTK apps seem to be considering sticking with GTK3 anyway ( to avoid libadwaita ).
https://linuxmint-developer-guide.readthedocs.io/en/latest/xapps.html