There are two ways to install GWENT: The Witcher Card Game:
1. Go to playgwent.com click PLAY FREE on the top and follow the instructions onscreen.
2. If you have GOG Galaxy installed, go to the Library, search GWENT: The Witcher Card Game, select it and click Install. Follow the onscreen instructions. The game will be downloaded and added to the side-bar on the left. Click on it when the installation is finished and press the Play button.