What if the winner doesn't respond in time?

Sometimes mails get sent to spam boxes. I've had it happen to me!

First of all, don't worry. Afaik when the gift is not marked as received after 7 days it will be treated as received. Do not give the key to any1 else, as you don't know, whether the first person didn't use it already, and the new winner will then have to mark the gift as NOT received. Don't panic and should the 7 days time period come to an end and your gift will be marked as not received just contact support.

Try to add him/her/them on steam.

Okay, thanks for the hints! He just marked it as recieved so I guess I started panicking too soon. Thanks all though, guess I'm prepared better for the next time