Website NerdLeaks has spotted a very interesting trademark and logo registrationÂ on the European Union Intellectual Property Office. Gwent: The Witcher Card Game might well be a standalone title based on the card minigame of The Witcher 3: Wild Hunt, which many have clamored for since the game's release. Some more details on the trademark application:
CD Projekt previously hinted to Eurogamer that they might announce something Witcher-related at E3, and this might very well be that announcement:
I'm usually pretty cautious with trademark applications, but in this case I'd say all signs point to this being real. Still, you should assume this is unconfirmed until the game is announced, as per usual.