I bought the key from g2a
In that case, I hate to tell you this, but you're screwed.
Nobody around here trusts G2A. Keys from there usually don't work here, and a lot of fake GOG keys are sold there. The chance you're going to get somebody to trade for it is next to nil.
I'd advise you look into trying to get a refund if you possibly can. If not, I'm afraid you're going to have to just accept it as a loss and move on.
Well, I already tried to get a refund and couldn't so now I'm here and I'm glad I came to the GOG forums cause I already found a trade. :)
Edit: I accepted the win and have moved on.