Recently Google found this for me:
Seems to be a wayland compatible key mapper, like the famous Windows AutoHotKey.
Seems to be written in Python, and while the screenshots look a bit strange, it seems to be GTK.
For old computers like the Amiga or C64 such configurable key mappers where popular, but I had not found something useful for Linux in the last 20 years. I think I was close some years ago, but them wayland appeared.
Well maybe there is even one more, see GitHub - snyball/Hawck: Key-rebinding daemon for Linux (Wayland/X11/Console)
I came to this again because someone just told me about https://qmk.fm/ an open source hardware keyboard project.
I have still to learn how to install key-mapper, maybe I have to delay it til winter. But at least it shows that some people do still use GTK for new projects.