If everything extracted you should be able to add the main .exe from the game folder to Steam now as a non steam game and use Proton that way (or do similar via Bottles or Lutris if you prefer).

Tricky part sometimes depending on the game is getting the right exe. I like bottles since you can do the Run executable to install which auto adds the .exe to bottles at the end, I needed this for Boltgun since I was picking a way wrong exe.