I found the solution
Good! but I think you must share your experience that if others are getting the same issue then, you can be helpful.
The Witcher 3 appeared in my library and I thought it did because I also have it on Steam (part of the transfer program GOG offers) but I couldn't play/install the game, it asked me to buy the game. Then I realised that I see the game in my library because I own the free DLC for it on GOG.