I didn't get free The Witcher from free Gwent pacage! How get the game?
I guess you have subscribed to GWENT Beta, you started GOG Galaxy and went to Gwent in your library. There was a banner in the tab, where you could subscribe to GOG's newsletter to get a free card keg and The Witcher.
All you have to do now is adding The Witcher to your shopping cart and once you check out the price should change to free. Go on and purchase it for free :-)