No response from the winner

Wait

It's a steam key... you can't reroll. Open a support ticket and try to contact the winner through Steam, and explain to them that they have to mark received.

When giving keys, I prefer contacting winners and giving them the key directly in the chat. That way they can activate right away. And don't worry, give him time to mark the game as received, he has 7 days to do so. Waiting is annoying but the 7-days rule is for the winner and the gifter.

Its been less than 48 hours. You are required to wait up to a week. At that point, contact SteamGift Support.

For now, contact the winner by adding them on Steam, ensuring they have seen and activated the gift and remind them to mark it received.

He never wins a giveaway before though he registered on SteamGift 3 months ago. Is it possible that he doesn't know he have to reply on receiving giveaway?

if the account owns the game, it counts as redeemed, you can ask admin after 1 week time if they havent responded to get your CV

wait 7 days before doing anything.

Meh. I opened a support ticket about this very thing a week ago.
They won't respond. Just deal with the loss.

Add him on Steam, waiting for him to accept.

BTW, you guys are really responsive, replying this thread so quickly and instructive. Good job, guys.

Everything is fine now. He accepted my friend request. I talked with him and he replied about receiving giveaway. He just didn't know that he has to respond on that.

Thank you, everyone.