Gwent, the collectible card game within The Witcher 3, isn't officially playable as a standalone title, but this mod within Tabletop Simulator is the next best thing.
It's multiplayer only — there's no AI for a single human player to take on. It's still a reasonably faithful re-creation of Gwent by the Steam Workshop user Mr.RiZZaH.
The mod is free through Steam Workshop, though Tabletop Simulator is itself $13.39, discounted from its usual $20.
In the official game, Gwent was updated with a neutral set of cards called "Ballad Heroes" with the latest patch this week. For more on Gwent, see this introductory video from The Witcher's official YouTube channel, below.