There is a Witcher Discord with a "gwent-lfg" channel which you can join to find other players for when the game hits in September for beta and also for other players looking for people to play other multiplayer Witcher games such as The Witcher Adventure Game

I can't seem to post links - so the invite code is Kk4s4Jx (do a quick Google to see how to use it)

There are several other channels on the Witcher Discord but you can mute the ones you don't want to receive notifications about.