I see you made a comment in https://github.com/ValveSoftware/gamescope/issues/1913 . Did you try vkcube like someone earlier in that issue? And the other workarounds like --expose-wayland?

Also what version of gamescope? And you don't happen to be running steam as a flatpak do you?

EDIT: they JUST released a new version https://github.com/ValveSoftware/gamescope/compare/3.16.14...3.16.15 . So if you are having issues downgrading might be worth a try, but I haven't been able to reproduce the issues you are having.

Also, I know it sounds weird for a linux machine, but update and restart just to rule anything weird out. I've had steam and steam related applications leave around weird state, processes, etc. before and restarting is an easy way to clear that all out and start fresh, eliminating variables.

I have the problem. I usually just downgrade one or two versions

Out of curiosity, what happens if you try to run gamescope from the terminal to just run anything (not even having proton in the mix).

For example:

gamescope -f -W 2560 -H 1440 -r 144 --force-grab-cursor --mangoapp -- glxgears

If this doesn't run, you should get some log lines at least to let you know whats on fire with gamescope.

Does it launch when you don't use any commandline arguments? If yes you can then add them back one by one to find which one is making problems.