The damper was not removed and the hole for the gas line was filled.Reading all of this reminds me of the saying, “I can explain it for you, but I cannot understand it for you”.
No matter what you do, this fireplace was meant to be decorative. Will not make any positive impact in heating the space. This is because of dilution air, exchange design, etc.
Question: How is your fireplace vented? Do you see masonry above the roof deck, or factory-built chimney (may be enclosed within a chase)?
Once converted from wood to gas, many units cannot be legitimately converted back. A hole has been drilled through the side for the gasline, and the damper section was likely removed. How exactly will your contractor address this?
Glass doors on units like this are not intended to improve heating efficiency. They are only meant to conceal the opening when the fireplace is not in use.
If factory-built chimney, gut it out.
Replace it was an EPA-certified high-efficient ZC factory-built fireplace with new factory-built chimney.
I'm done posting in this thread.