There are three ways to install GWENT: The Witcher Card Game:
Go to playgwent.com click PLAY FREE on the top and follow the instructions onscreen.
In GOG Galaxy select the Search Icon and search for GWENT: The Witcher Card Game. Click Buy and Install the game. The game will be added to your library and once installed can be launched through the Play button.
Go to the Store in Steam, search GWENT: The Witcher Card Game, select it and click Play Game. Follow the onscreen instructions to download and install the game. Once the installation is finished press the Play button.