If you buy a game on GOG.com and find that it doesn't work properly on your system, and our support cannot fix the problem, you get a full refund. It's a worldwide guarantee, and you have whole 30 days after the purchase date, to contact us about the refund.
If you bought a game by mistake, or simply changed your mind about a purchase, you can get a full refund within 14 days, as long as the game wasn't downloaded.