Won this giveaway

Just mark it as "Not received". Have you tried contacting OP on Steam?
If there's still no reply, then just report to to SG support.

a) check your spam folder (also make sure you're checking the email associated with steamgifts)
b) it would be wise to unlink the giveaway, you're not explicitly calling him out for breaking any rules perse, but still, it can't hurt
c) mark it as not received if you still don't see it in your spam folder or anywhere else for the time being, as it has been a week
d) try contacting him through steam
e) if that doesn't work, just leave it marked as not received and forget about it :(

Based on who it is that ran the giveaway odds are that they think they sent it out already. It likely did go to your spam if it was sent. Are you sure your steam email is the one you are checking? I'd try adding on Steam as others have said, and then ultimately hit the sad button. Seeing the neg probably will make them act to correct it.

I just had a look at your profile and saw that a game was missing (not activated), maybe he asked for a reroll based on that ?

please disregard my comment if I'm wrong.

I have an idea about why it might be - A Game of Dwarves you won have an odd tendency to not show up when you look for it in people's library unless they played it in last 2 weeks (the reason why SG screws up so often if you try to enter giveaway for DLC). Maybe he assumed you didn't activate it and is waiting for reroll.

Worst body part EVER!