You can play cross platform, but only with a random stranger. You can't find your friends so the feature is meaningless. It's also not the game from The Witcher 3. They made it too complicated to try to sell microtransactions and in the process, ruined the game. Don't call it Gwent if it's not Gwent. 0 stars, and I'm going to print out a paper deck to play the real game instead.
