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argh... if he doesn't answer you at all, the one thing you can do is wait for 7 days and mark the game as Not Received

Patience.
Keep trying to contact him, either through SG on an old GA, or add him on steam and explain the situation.
The key, however, is to be patient and understanding. People make mistakes. Good luck getting it resolved soon. :)
If he doesn't get it resolved within 7 days, mark the giveaway "Not Received".

Same thing happened to me. I tried a couple times over Steam and by e-mail, but still no response. I just had to mark the game as not received. Sometimes, it's just an honest mistake. Be patient and a wait a couple days to see if they respond at all. Good luck.

The best thing to do is post a comment under the giveaway entry. People can miss emails and may ignore Steam add requests by mistake. Posting in the topic also makes it easier for support if they need to intervene at some point.

well it happens comment on his page give him time

Once I have this problem. I contact with creator and explain situation, he no got idea what happend and what to do. I advise him to contact support and ask to delete this giveaway with my permission. Support contact with me and I agree to delete it. Problem solved.