It may be just my laptop, but I can only see the top foot or so of your fireplace, and a lot of wall above. I'm going to try to post your pic below, so more of the fireplace is visible.

My only concern would be the marble hearth. Will you just put furniture over it so it can't be seen?

I would take the marble off of the wall and floor - make it look like it was never there. I am concerned that there is something I am not considering in blocking off the opening.

Perhaps call the company that supplies your gas and have them check out the unused FP and see what they have to say. You'll be able to match the floor boards where the hearth is?

New floors going in.

You’ll want to have a professional in to safely close off/insulate the flue opening and firebox before closing it into the wall.

edit - just re-read and noticed they share the chimney. Removing the advice to cap off the chimney! :)