region lock?

Not 100% sure but I think it says that as long as they accepted it the gift, whether they activated it or put it in their inventory.
Also I assume it is for you Space Farmers giveaway, I guess the gift you sent is an extra copy that you got in your inventory after activating the game on you account. Generally these gifts that popped in your inventory are region locked if you are not in a ROW region.

I'm not 100% sure about this, but as far as I can remember space farmers 2 packs drop an extra copy that's region locked but has no mark, I recall a discussion on SG about the same problem previously but i'm unable to find it sadly.

Ask him to send you the gift back and either try getting another copy (possibly a bundle key, it's been bundled 9 times already) or just take a hit to your gifts delivered ratio, if the winner does not send the gift back I'd say there's no proof whatsoever that the winner is telling the truth about it being locked so the best course of action in that case would be to submit a support ticket to have them mark it as received since you can proof that you did send the gift already.