Winner, not received the gift.

Add him to friend and ask about the cd-key , if he reject / doesn't answer , check on support

Because it can be that the cd-key is unused , and when you will try to do a new winner , he will use that code

Keys are risky. Just re-roll but there is no guarantee that the key will not be used.

If he refuses to use it, reroll, and give the key to that user, if that user says the key is used, report the other user and delete the giveaway.

Add him, like other said. But the support doesn't reroll key giveaways, they can only ban him when you provide a proof that you sent him the key. You won't see the key again.

People have already told you what to do with this giveaway (contact the winner first, the possibly the SG support), but I'd like to give you a friendly advice for your possible future giveaways (hopefully you haven't completely lost your trust to this community). If you're gifting a key, your best bet is to add the winner in Steam and give them the key in a chat. That way you can ask them to activate the key immediately and you can avoid being scammed much better. I feel sorry for you pal, this is just why I prefer gifting games from my Steam inventory rather than keys, there's always certain risks with keys. :(

You will need to try and contact the user through steam to ask him what's up. Maybe it's simply that he received your mail in his spam folder. If he doesn't respond / ignores you or so, contact support. They will ask you to prove you sent the key. If you did send it and can prove it, you're fine, don't worry.

I think you need to get his IP and visit his house with an axe in your hand and axe him why he didn't mark it as received!:)

same thing is happening to me, well, i guess i should start to send cd key only via steam chat to winners

Ok thank you all for the advices I decide to write to support. Topic is colsed now.