Tell the person you didn't get the gift, report if he doesn't give anything.

that happened to me with my payday win. He gave me shadow grounds as a sorry gift. great guy actaullly

Was it a steamgift or a key?

steamgift

Sucks, my first win was a dud too. Apparently the creator didn't know that you had to actually own the game you were giving away.

Happened to me with Beat Hazard, and I want the game so bad.

This is why keys bundle keys shouldn't be allowed

Happened to me once. Told the guy and he offered up the key he was gonna use for himself. Didn't want to take something he wasn't planning to offer so I just refused and marked it as sent since he was such a nice guy and it wasn't his fault.

If it was a steam gift, all you have to do is spam the "add game to library" button to death. It works all the time.

You know that marking fakes as received can get you suspended/banned if someone notices you either:

• Not redeeming a gift you received (meaning you can have won it for trading or giving to someone else), or
• Winning twice the same title?
  No point on being nice for someone who don't deliver giveaways he created and putting yourself on the line

As for OP question, if the gift is in your inventory and you're trying to redeem it, it may be just an inventory glitch. Try refreshing (F5) the inventory(and reddeeming the gift) insistently, even restarting steam in the process if you need to. Sometimes it even happens with stuff you bought yourself and never sent to anyone. If you have someone you REALLY trust, send him the gift so he returns it to you (as long as he doesn't redeem it himself)