http://www.pcgamer.com/star-wars-battlefront-2-multiplayer-restored-now-with-steamgog-crossplay/

Star Wars: Battlefront 2 (not to be confused with Star Wars Battlefront 2) lost its multiplayer support in 2014 when the GameSpy matchmaking it depended on shut down. That loss of "official" online play was a big knock against the game, since the multiplayer was what made it stand out. But as of right now, that's no longer a concern.   

"Star Wars: Battlefront II (Classic, 2005), the critically acclaimed shooter of massively entertaining warfare, has received a critical update," GOG announced today. "Powered by GOG Galaxy, the multiplayer function of the game has now been restored AND with added Crossplay support between GOG Galaxy and Steam."