What to do when a user activated a gift but did not acknowledge it on steamgifts?

Exactly.

Giveaways:

• The submitter and winner(s) of the giveaway have seven (7) days to complete the gifting process. This includes the activation of the gift on the Steam account of the winner.

Yea, if the receiver has the game on his steam profile, support will most probably just mark it as received. If this is not the case, it might be quite complicated...

Shame that some gift receivers do not acknowledge the receipt in a timely fashion since that is what completes the process for the gifter getting credit.