I was a new trader on GOG, looking to trade my Witcher 3 for keys or GTA V. Since I was new, I went first, I had no rep to speak of and the trader did.
His steam name is MattAttack43 and he offered 20 TF2 keys. I was given "Bad Rats." If any of you come into contact, beware. And if any one knows of any recourse I can take, please help. It was my first trade and I followed the correct protocol, I believe. So, let me know if there is anything to be done.
It would help to warn others if you can also post the GOG username of this person who scammed you in this thread: http://www.gog.com/forum/general/known_scammers
And to avoid this user to use your key for activation or selling/trading, you can ask GOG Support to void that code for you after explaining the situation, although this may take time and help is not guaranteed, since this wasn't GOG's fault. With proof, GOG can null that game even after it was redeemed.
Unfortunately, the chances to get a valid code back is practically zero. You may want to try first if you can redeem that code for yourself or give that code to a friend of yours in case is still available to redeem. The sooner the better. And if it was already redeemed, I think you can see who GOG user did it in the very same page to redeem GOG codes: https://www.gog.com/redeem
It's really unfortunated what happened to you here :(