Try New Technology

Below are the basic steps:

1. In Steam use the “Add a Non-Steam Game to My Library…” to target the installer file (.exe).
2. Before launching the game, right-click on the game and select Properties.
3. Compatibility tab select “Force the use of a specific Steam Play compatibility tool” (doesn’t look like you can change this after the fact)
4. Close the Properties and run (Play) the installer. The game should fully install within Proton (D:OS2 DE had an error at the end, but still ended fine and runs).
5. Use the Properties menu again to set an icon (.png or ,tga) and name the game correctly (ie “Divinity – Original Sin 2 – Definitive Edition (GoG)”)
6. Enjoy!

EXTRAS:
If you need to access the “C” drive of the Proton install of this game the folder location will be in “~/.local/share/Steam/steamapps/compatdata/[appid]/pfx/”

• NOTE: If you need to put any save files inside of the Documents folder, then use the “steamuser” (ie “~/.local/share/Steam/steamapps/compatdata/[appid]/pfx/drive_c/users/steamuser/Documents/”)

If you need to load something else into the games prefix (ie DLC, Patch, etc.) then you can do the following:

1. In Steam use the “Add a Non-Steam Game to My Library…” to target the installer file.
2. Before launching the game, right-click on the game and select Properties.
3. Compatibility tab select “Force the use of a specific Steam Play compatibility tool” (doesn’t look like you can change this after the fact)
4. Close the properties
5. Open Properties again and this time add the following to the “LAUNCH OPTIONS” field: STEAM_COMPAT_DATA_PATH=”~/.local/share/Steam/steamapps/compatdata/[appid]” %command%”
6. Close Properties (mine wouldn’t close, but I could still click the Play button and the DLC installer I was running worked fine)
7. Remove the install from your Steam Game Library list

NOTE: This was on Manjaro KDE running the latest version of Steam. I would expect it to be mostly/exactly the same on other distros.