IChampSI: I wanna try to play Diablo and Hellfire on my Fedora 33 machine using Steam Proton. You have to configure Proton explicitly for the game, otherwhise Steam would use its internal Linux library. The latest Proton version breaks non steam games, so Proton 5.0 has to be used.

The first issue that occurred was, that GOG installs the game into the directory "GOG Games" and Steam does not recognize the directory name if there is a whitespace. So steam thought the directory is "GOG" and everything behind is a launch argument. Long story short, I changed the directory name to "GOGGames" and Steam was able to start the launcher.

To get log messages, I've started Steam in my command line, but I was not able to find a solution for the following issue:
When starting the launcher, I can choose "Diablo", "Diablo Classic", "Hellfire" and "Video Settings".
- If I click "Video Settings", nothing happens.
- if I clck any of the other options I just see this message in my Terminal:
Game removed: AppID 0 "Diablo.exe", ProcID 49526
Uploaded AppInterfaceStats to Steam

So I tried starting Hellfire or Diablo instead of the Launcher, and I got this error message:
[0205/113434.288925:ERROR:frame_sink_video_capturer_impl.cc(206)] Invalid resolutions constraints: 0x0 must not be greater than 0x0; and also within media::limits.
>>> Adding process 57986 for game ID 33554432
Game removed: AppID 0 "Diablo.exe", ProcID 57925
Uploaded AppInterfaceStats to Steam

So it seems the wrong resolution is configured. I can not access the video settings, but I configured the resolution in dx.cfg.exe, but the issue persists.

Could someone please help me to find a solution?

Anders_Jenbo: Why not just use DeilvutionX?
https://github.com/diasurgical/devilutionX/releases

IChampSI: Because I want to find a generic way to run GOG games on linux. If I get Diablo working, it's probably possible to get other GOG games to work using Steam Proton.

But thanks for your link, I give it a try, even if I will continue my efforts to get Diablo run via Steam Proton.