Game sent, redeemed but didn't checked as received

Contact the winner and ask him to check ?

He has a week to mark it as recieved. If he hasn't done it then, contact support.

Try adding him on Steam and messaging him. If you're polite there's no reason why he wouldn't be, unless he's just an ass.

Wait until a few days are left until the week deadline is over, then contact the winner and let him know about it. If he doesn't mark as received at the end of the week, contact Steamgifts support.

This

Call a hitman

You should had send it to me, I would mark it immediately. Joke aside, you should contact support if he didn't do anything within 1 week.