

since it looks like you’ve already activated the dropdown, this is a bug, probably on Heroic’s end. if you don’t see anything other than Linux in the version dropdown, I would put in a bug report on the github.
but since you own the game on GOG, you can just download the installer yourself and add it through Steam or Lutris or Bottles or [insert your preferred method]
oh that’s a good point, I didn’t consider that game mode might be causing the problem. I guess I always install from desktop mode anyway, so I obviously never ran into problems with heroic in game mode.