If you didn't get the key, don't mark as "received" but mark "Not Received".
If you mark as received and he didn't give the key (so you can't activate), admin will suspend/ban you.

And don't regifting!
They will suspend/ban you also.

The other consequences if you didn't have the game in your library is you can't enter protected giveaway,
better games usually protected by SGtools, you can't enter them if you have unactivated games.