I read that you can register your Steam version of TW2 here on GOG. Okay, maybe I am stupid, but how can I do that? I clicked on "redeem gift" in the footer and entered the key that I have in Steam, but all I get is "invalid code". So what am I doing wrong?

There's a special page for it:

http://www.gog.com/witcher/backup

Erm... I knew I would look stupid. :)

Thanks for your help!