A lot of these games that have Linux native versions, official or otherwise, that aren't published on Steam are supported by Luxtorpeda. It can easily be installed through the ProtonUp-Qt tool and then after restarting Steam it will appear in the list of compatibility tools. Just set it on any supported game and the Linux version will work much like it came from Steam.
In this case, when launching the game, it provides options between ET: Legacy (32 or 64-bit) or ETe (32-bit).
One exception I've noticed is that the project doesn't support reverse engineered ports such as OpenJKDF2 (Jedi Knight: Dark Forces II) since it's legally questionable. Not an issue here since the original engine was open sourced.
Luxtorpeda:
https://github.com/luxtorpeda-dev/luxtorpeda
Supported games:
https://luxtorpeda-dev.github.io
ProtonUp-Qt:
https://davidotek.github.io/protonup-qt