Azrael360: I'm a newbie at this, I have made only 6 giveaways and been registered for only 2 weeks there, but I was starting to think this could happen someday, because the system seems to be very protective with the participants but not with the gifters, they have to just forget to mark the "gift received" spot and you'll end with one "gift not received" in your profile, even if you did.
It happened to me recently, I sent this dude the gift but he started ignoring me after redeeming it and never feedbacked me at SG. 2-3 weeks after my GA was over I simply opened a support ticket explaining the situation and showing proof that the winner received and redeemed the gift I sent. Eventually they replied saying they would send it to the SG big boss to adjust my feedback manually, but that it would take a while since he also had other requests in queue. At some point after I got my feedback.
mondo, try googling his Steam username. Like this: site:gog.com username . Maybe you can find out his GOG name that way and contacting him through here. Also try to contact him through Steam like it was suggested. Sometimes SG gift emails end up in the user's spam folder, other times the email is simply wrong.
Edit: almost forgot, I hear that if the nature of the gift is a Steam key, there is a chance the support won't reroll the GA even after the 7 day period, since you can't know if a key has been used. Even if the game isn't activated in the winner's account, he could've been extra douche-y and gave it away to someone else, or traded it. I'm not saying that is the case here, but it has happened before and that's their reasoning for such decisions.