GIMP’s GTK3 port was finished several months ago. What remains to be done for GIMP 3.0 is bug-fixing and porting to the new Plug-in API.
The best way to upgrade to GTK4 is to upgrade to GTK3 first. There was some talk about working on GTK4 soon after GIMP 3.0 is out, but whether that will happen or not is uncertain.