ChrisSZ: The key should be in your account:
https://www.gog.com/account Then select the game and check the "More" section.
ZFR: I don't think he'll understand you. Let me try:
go to account!!! click more!??! you'll find it ther!!! lol!??! wtf?!!!?
:D Don't be cruel ;) ;) Anyway ... this question gets asked again and again (I personally won't respond to these questions anymore. If someone is not able to use a search engine ... ).
I answered the same question from a different user with rep 0. New user. And they didn't even send so much as a 'thank you'. Wasn't too important, I guess.
So ... I don't think the OP will respond and send a 'thank you'.