He probably gave you a key that was bought from a gray market with a stolen credit card. GoG noticed and pulled the game because it was stolen merchandise.

I feel your pain, but everyone should realize that trading on GOG is not even allowed -only merely tolerated.
Since they have never implemented (and have no interest in) a trading system, GOG never cared about what anyone does with their purchased game codes, and they have no means or reason to verify if a trader is trustworthy or not.
Alas, any trading activity is at your own risk, and in my opinion trading here (especially without knowing the other part or having no mediation from regular users) is almost always a bad idea.

Maybe next time don't violate the terms of service and don't trade with people, risk vs reward and all that.

Don't accept cd-keys from random people. You never know where or how they got them. It's never Steam/GOG/Origin/Uplay/etc.'s fault that you made an error in judgement with a person. If you want to trade safely, you need to take precautions for all of the dangers. That includes revoked keys.

Wasn't bloodrayne on sale on the last sale for like 75 cents?

I really don't see why you'd trade 1:1 on that type of game.

Maybe you should buy a brain. Gifting is not against the TOS.

I don't know if this is because you are Russian but Gifting and trading are two different things. The op has stated he is trading not gifting. Try and reply in a mature manner next time and don't cut yourself on those edges.

Well, the Russians are known for their "generous gifting" if you know what I mean :P

Does sell in the statement above include barter? I've found both financial and legal dictionaries that state that selling involves monetary compensation, and that barter differs from sale. That said, definitions across the internet are certainly not consistent. Genuine question here. Does the above forbid an exchange of gift codes or not?

edit:
The Steam TOS is explicit about the differences and uses expressions like "trade, sell or purchase" and "trading and sales".

I would say yes because of the way it was worded. The second part says can't charge money and that's clear cut on what it is. But the first part says you can't sell them as in I have a copy of a game and you give me two copies of another game to make for the loss. It's vague and a blanket phrase imo.

"But for me, it was tuesday".

In that case contact GOG support and tell them to revoke the key. If he said you didn't send him MDK2, then according to what you're saying he's either lying or he didn't get it, so you should have your own key back.

Also check the scammers thread and GOG wiki, both have lists of people to look out for. Search for "scammer thread" here.

Write to GOG support and ask them what the reason for revoking the game was. We've seen several cases of credit card fraud and revoked keys from gray market sellers before, but they were all about new games with high price points. Bloodrayne was available for $0.80 in the last sale and even had GOG keys in a Groupees bundle. Seems strange to me that someone would use a game with such a low demand for any kind of fraud.

And try this link: https://www.gog.com/account/refresh (maybe it's just a bug)

Dude,chill. He's the forum's tough guy.