Star Wars Battlefront II supports crossplay between Steam and GOG.com. No, not that one. The original one from 2005. See, this is what happens when you don't name your games clearly.
The original Battlefront II, also known as Battlefront II Classic, is one of the many good old games you can get from GOG (get it?) and GOG Galaxy has now restored the multiplayer functionality to the game, which is pretty cool.
The multiplayer was originally shut down back in 2014. Now it's not only back up for the Windows PC version of the game, but there's cross play between Steam and GOG. This should help to keep the fights fully populated. What a time to be alive.