Proton is just Valve's fork of Wine. It had a lot of game-specific patches, to make all the Steam games work better.
Wine isn't meant specifically for games - you can run most Windows applications in it. It's just translations of Windows syscalls to Linux equivalents, to put it simply.
That's not Baldurs Gate, it's Baldurs Gate: Dark Alliance which is a completely different game that was on consoles. It was an Action RPG as opposed to an RPG.
But it was also great fun. Especially with a friend.