Hey!
I'm sorry for replying so late.
In the meantime, I reached out to support, but it seems that it will take a while before I hear back from them.
Anyhow, I don't see that you have this game on your SteamGifts account. Would there be any punishment for you if you just accepted this? As long as it's on your account and not doubled here and you do have the DLC, it should be fine I guess? I could dispose of the key to ensure that it can't be regifted. That would be my suggestion for resolving this as you already saw the key.
Other option besides waiting for support, for me, would be to activate the key myself and request the removal of this GA.

Either way, no problems with me :)
Cheers!