Won key for different game than listed and already have the game

Wait for a response. If things won't work out contact SG. Do not mark as received if you did not receive the game stated

Don't mark it as received.

Leave a note in the giveaway explaining to the giveaway creator that it's the wrong key.
The creator will either replace it with the proper key or, if you're okay with it, he'll ask support to delete the giveaway.
If neither of those happen within 7 days, mark the giveaway not received.

If it's not the right game as listed, tech it's not recieved. I would contact who hosted it and wait to see if he has the correct key. Either way you should not mark it as received, give him at least 24 hours to reply though to be fair.

Contact the creator (you can comment on the giveaway) about this, and let them sort it out.
If that doesn't happen, mark the game as not received after 7 days passes (and the box will get ungrayed)
Don't mark the game as received until you get the expected one!

No matter what, do NOT mark it as received, or you will be punished for his mistake.
Wait for him to respond and fix the problem either by obtaining a copy of the correct game or negotiate with support about changing or deleting the giveaway. Until then, don't do anything (other than contacting him which you already did).
If no resolution has happened after 7 days, mark is as not received and change it later if by any chance he resolves it with you.

For future reference, whether you had the game or not is irrelevant - even if you got the wrong key and spent it, you are still entitled to the correct game (sender's loss).

Try to talk to the giver within a week, ask for the correct key or if you want, create a ticket to delete the giveaway if both of you agree. If you don't get a proper response after 7 days, mark it as not received, otherwise it'll only be to your disadvantage and you will get suspended for not redeeming the game.

ok I will leave a comment and wait. :)

He can't regive it since it's not the one that he mentioned. Just contact him, if he won't give correct key, contact staff to give him suspension and a warning, so he won't post free giveaway keys masked as some other games next time.