Stardew Valley, the lovely farm-life sim from ConcernedApe and Chucklefish recently gained the long-awaited multiplayer support. The Linux version was previously left-out on GOG, but they just added it.
To access it on GOG, find it in your "Game Goodie" section of your library when you click on Stardew Valley:
You can connect to others via IP address (GOG say it's tested and works across LAN too), since GOG still haven't put out Galaxy for Linux and there's no hint in sight at when they will.
It's good to see GOG moving to allow more games to have a Beta version up, in addition to the main stable download. Previously such features were reserved for Galaxy, as one such user found out and posted about on reddit.