I hope you're aware that you need to either a) have never downloaded the game or b) have experienced issues when trying to run the game and it doesn't run after you've downloaded it, or chances are you won't get a refund.

GOG tends to be very sympathetic towards their customers, but don't get all mad at them when you ask for a refund simply because you didn't like a game and they refuse to refund you, because that's not a valid reason for them to refund a game.

Also, obviously, they don't refund games that were gifted to you, you have to had bought it yourself. And, even after you downloaded the game and tried to run it, they'll try to help you run the game; only if they fail in helping you getting the game to run will they consider a refund.