User have received and activated the gift, but he didn't marked it as "Received"

Contact support

Wait till the seven day term is up. Then submit a support ticket if they still haven't marked as received.

Contact user first, if no response, contact support and ask them to mark it as received.

http://www.steamgifts.com/about/guidelines

Winners should correctly mark their gift as received or not received within one week of the giveaway closing. This feedback should be kept up-to-date if the status of the gift changes.