People not responding to gifts sent.

tha'as y contacting via steam is a better way to deliver gifts
Try to contact him and ask to mark as received, otherwise show proofs to support so they will mark it

If the winner has the game in the Steam library, submit a support ticket about the situation with a screenshot or a link to the user's library with the won game, and a staff member will take care of the feedback for you.

Edit: Ah right. I assumed that you've already contacted the user. Try to contact the user first.

Send a friendly reminder, if not marked after 7 days, contact support.

First, add him and remind him to mark as received. If he blocks you and still doesn't mark as received after 7 days, contact support and explain the situation.

Contact him via Steam and ask him about it. He might be new and didn't know about this. Its happen for me months ago but when I add him on Steam and tell him about the gift and that he must responded with received, he did it right away.