Can't answer your question but there are a couple of Witcher codes available in the "Take-one-give-one giveaway thread". You will need to leave a game in exchange, but any game will do (a surplus game from a bundle for example).

Thread can be found here:
http://www.gog.com/forum/general/takeonegiveone_giveaway_thread/page1

List of games can be found here:
https://docs.google.com/spreadsheet/ccc?key=0ApRZKS-zv1o1dE15ZldBZ1F5Z1psc2RDRFZPUkJTRUE&usp=sharing&richtext=true#gid=0

Hope this helps.

Thanks for the info.
But how does it work?

I would buy a game Master of Orion 1+2 with my redeem code and then quote the one who has the witcher that I will leave my game and want the Witcher. Will I have to wait until they send me the game?
What if they don't respond?
And if I leave my game and don't look through my messages and don't see a response from someone who wants my game?

EDIT: Seems like it's not possible to redeem gifts with these codes