I fear i haven't got the correct game in a giveaway and may get banned

I got a free key from GMG, and getting reported as owning DDE when I visit the page for it.

First, right click Mafia II in your library and select properties, then go to the DLC tab to see if you own the DLC that comes with the Digital Deluxe Edition. If not, It's courteous to try and contact the gifter to give them a chance to send the correct gift, then mark as not received if things don't work out.

In that case, it means that you did not receive Mafia II: Digital Deluxe Edition, but Mafia II instead and the gifter most likely tried to exploit the sg system and get CV( Mafia II was given away for free a few months ago).

What you should do is: Mark the gift as not received after a week passes, and in the meantime submit a support ticket and they will be able to change the giveaway and make it reflect the truth.

Good luck.

Well, support ticket!

Everyone else has answered your question, but for the sake of clarity, ...

1) Contact the gifter. Explain that you have not received the gift advertised in the giveaway. Give the giver a chance to fix it.
2) Contact Support and give them a heads up on the situation.
3) If, after seven days, the gifter doesn't sort this all out, you will mark "not received," and Support will deal with the rest.

Even if you checked "received" you can go back, and click "not received" later, if you have a reason. (As you apparently do.) Make sure first, and then contact the gifter, if he/she is unwilling to come to an agreement, contact Support, that's my advice.