It's a card game.
Gwent Novigrad expansion

The second expansion for GWENT: The Witcher Card Game is now available for everyone. Novigrad, as the second expansion is called, introduces a new faction, new cards, new leader characters, and more.

There are over 90 new cards including 15 cross-faction ones included in this expansion. The new faction is the Syndicate. They sound kind of evil, right? There are also five new leader characters. Novigrad also adds in new abilities and gameplay mechanics to play with.

This expansion is a free content update for the game. It's available for the PC via GOG, PlayStation 4, and Xbox One versions of GWENT. If you log into the game before July 17 at 11:59 (CEST), you will receive a free Novigrad Keg. This will grand you five cards from the new expansion.

CD Projekt RED also reveals that for a limited time, you can currently hop into a special Arena Mode event that lets players draft from the Syndicate cards. There is also an expansion themed promo for The Underworld Pack that you can get in the in-game shop. I have no idea what is included in that or how much it costs.