mondo84: If you purchased a game on GOG recently (e.g. within the past 30 days), and you haven't downloaded that game from your shelf, it might be possible for GOG support to remove the game from your shelf and provide a gift code that you could give to someone. However, if you've owned the game for awhile or if you've downloaded it, then that won't be possible.
I'm pretty sure the 30 days is for if you're having problems running the game. You can only get refunds of games on your shelf you
HAVEN'T downloaded within 14 days from time of purchase.
EDIT
Source:
http://www.gog.com/support/policies/terms_of_use "A sale is considered final either 14 days after purchase OR when you make a download attempt for a game or its bonus content."