Just got a steam deck and was adding some of my gog games to my steam library. Was able to add games initially (added thief 2 and Deus ex) then all of a sudden it has stopped worked. Either having the "add games automatically" setting or individually adding the games with "add to steam" does nothing. This stopped working after I added T2fix mod as I wanted to add fan missions. I added the exe to the install of Thief 2 then added t2fix as a game to heroic then used the "run exe on prefex" to start the patching process. I then removed the T2fix game I created along with its prefex folder (I'm very new to this so still figuring out linux).
Since then I've not been able to add any games to steam at all, adding them and rebooting steam or the deck just gives me the option in heroic to add it again which does nothing. Additionally adding the exe via steam doesn't seem to work either, though it works with other apps like gimp and blender.
I've also reinstalled heroic launcher and deleted any prefixes files that were left over, along with deleting user settings via discover.
So I don't know what's left to do, clearly I'm missing something or need to enable an option.