Gwent has been available for PC through GOG as well as for mobile platforms. It was not available on Steam till 19th of May.
Gwent: The Witcher Card Game is a free-to-play game completely though, there are some in-app purchases through which you can purchase an extra pack of cards in order to speed up you building up of the deck. The Steam version has a complete cross way for the mobile platforms and GOG. So, there will be no issues in finding you opponents.
Gwent is a single player 1v1 game. It is also a cross-platform Multiplayer game and support controller partially. The genre of the game is RPG and Strategy. The developer and publisher is CB PROJECT RED. Gwent is a card game. You can collect and use Geralt, Yennefer and other famous Witcher-world heroes. Build your deck with spells and special abilities that will make you win the battles. You will have to make clever and spontaneous decisions in the game. You can fight in classic, seasonal and Arena modes.
The game seems to be interesting and you can definitely give it a try.