Roadmap shows GTK+3, why not GTK+4?
BIG RED FLASHING WARNING: I have done very little with GTK ever!
Other than a few things related to clipboards, I believe I have a compiling version of gimp. I do not know if it functions. I do know spacing of packed objects will be a bit off, due to GTK no longer allowing setting borders, etc. in the same way.
I have used github, but not gitlab. I would be interested in working with someone to get gtk4 completely working.