I'm noticing that GOG has a severe lack of games that support online multiplayer. Even the ones that do support online multiplayer seems to be dead. It's too bad though, I love playing online multiplayer.

Apart from Gwent, what other games on GOG have active online multiplayer communities?

I believe No Man's Sky can be included in the list, since there will always be players present whenever you visit the hub where you can look for quests and things like that. That said, if what you're referring to as an active online multiplayer is one where the community solely comes from GOG alone, then I think there really aren't that many options. I say this because even No Man's Sky's high online players' count relies on its crossplay between Steam and GOG, and even across platforms too I think.