Azrael360: It looks like he hasn't been online since 6-7 days ago on SG and Steam, probably from before your giveaway ended. I don't know about here but maybe he hasn't been able to be online. I hope he's okay, but you should give more time to him until he's able to be online and mark your gift as received. Hopefully he can do that sooner than later.
siulebuo: I am OK with this, but do you know if there are a time limit to mark the gift as received / not received?
The giveaway creator has 7 days from the moment the giveaway ended to send the gift, in the meantime the winner, after 7 days have passed since he/she won, they will be able to mark the gift as not received (before that he/she can mark it as received from the moment they won). Once 7 days have passed since the giveaway ended and the giveaway creator has noticed that the winner hasn't been online or hasn't marked the gift as received (knowing that the Steam key or gift was good and already sent), can contact SG Support to notify of this situation. Support can mark the gift as received for you, or reroll if necessary.