awhyloo: If you have a Steam copy, you can redeem a backup copy on GOG. The same goes for if you also have The Witcher. You just have to look for the product key on Steam.
I can't post links, but it's at gog.com / witcher / backup
MistNuts: OK I just can't find my Witcher 2 Steam CD Key. There is no option for that. How do I find it? I still have the e-mail receipt, if it matters.
With the e-mail receipt you could, theoretically, by
contacting GOG Support, sort it out with them and prove to them you have, in fact, rightfully purchased a Witcher 2 Steam copy, and they'll end up backing up your game. People have done it before, so, yeah, it's possible.