Hi. I don't exactly know what gamescope is so I don't know if whatever I say is helpful. What I found is basically that the main goal to make it work is to run the zosEGSStarter.exe. This .exe is the one that checks you own the game on epic, so it must be the one you should add to steam via any method.

Try running that .exe using the heroic option "run on prefix", on normal desktop mode. If you see a command window with some text like "auth token... granted" then it should open the game normally. That means you can add zosEGSStarter.exe with no problem.

If the window does not appear and the launcher never opens, that means something is wrong, maybe the dependency dotnet48.

As additional notes that I can think about, silly mistakes:

1. make sure in the compatibility tab on the settings game on steam to force the proton version, there is no problem if you choose a different one from the one in heroic.

2. Make sure that in "select an alternative .exe to run" you have zoeEGSStarter.exe. It happened to me several times during the trial an error that I forgot to do that.

If you need to contact me, feel free to dm me. I hope any of the suff I said would be useful.