Interesting. I haven’t had any issues with gamescope for a long time… Back when it was new I definitely had issues running things with it… But it’s been a long time since that’s been the case. I can recommend running steam from a terminal and viewing the output after trying to run the game with gamescope. It might point you in the right direction.
In my experience this is almost always the compositor not properly syncing display frames with the game render frames. The best solution I’ve personally found is to run the offending game through game scope. It worked for Fallout 4 on my rx 7600xt gpu when I went to lock the fps to 60 (to avoid physics bugs, stupid Bethesda). Without game scope locking the frame rate caused horrendous stuttering despite solid fps.
Weird! I’m running Bazzite kde (so fedora based, like Nobara but with different tweaks and it’s atomic), and gamescope gives me zero problems. Might be some weird combination of software versions between gamescope and kde in your system. Or just a general kde config problem. I love kde, but all that customization can absolutely come at the price of stability at times.