Gas Fireplace Installation

I have been looking at installing a gise fireplace unit and looking for tips advice etc.
The house already has a mantle and gasline from where one of those old porcelin type heaters used to be. So, what I have to do is remove the brick from the old heater and insert one of the gas fire box units into the space.
I have been told by many places, that they can hook the unit up for me but will not do the constrution/demolition.
Has anyone had to do this before? Suggestions? Thoughts?
Also, because I already have the mantle etc, the fire box sizing is going to leave an inch or so, of gap between the unit and the marble. Thoughts filling the space?
The way that the mantle is set up is that is that is would with a would frame, then marble frame inside the wood, then the foe bricks. The sizing inside the marble will leave the space. I was thinking of just using trim pieces that would match the mantle to fill the space. Of course, the question becomes how to secure the mantle?
Thoughts anyone?

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