If you want to use gamescope you must make sure of the following things (as of writing):

1. you installed gamescope from Flathub and its runtime version is the same as the one used by Steam (can be done and checked with the “Warehouse” app). At this moment Steam uses 25.08
2. for games that use Proton you MUST install Proton-GE as a Steam flatpak Add-on (from your App Store) and then enable it in Steam under Settings -> Compatibility -> select the one with “(flatpak)” or under a game’s Properties window. Otherwise, gamescope will have swapchain errors or crash. Even the builtin Proton versions don’t work as it stands. (upstream issue)

Gamescope can than be used as a launch option.

An Example: Running “Road to Vostok” at a lower resolution (because it doesn’t have a setting for that inside of the game sadly, and my hidpi laptop can’t really run it too comfortably at native res) + MangoHud(installed as flatpak):

LD_PRELOAD= gamescope -f -W 1920 -H 1200 --mangoapp -- env LD_PRELOAD=“$LD_PRELOAD” %command%

Keep in mind some flags are not working on wayland like -o (allows changing refresh-rate if the game is unfocused) (ref)

edit: seems like there’s also a bug with fps lagging after ~20mins or so, which can be mitigated with prepending LD_PRELOAD=“” to the launch command + adding env LD_PRELOAD="$LD_PRELOAD" before %command% (upstream issue)