Why are you putting in a gas insert? why not just leave what you have and do a gas log? the bricks in the arch are supporting bricks for the rest of the structure instead of screwing w/the bricks, remove the whole thing and do your insert, surround it w/something diff modifying the arch and sticking in something different isn't the best option. (is a fisheye lens all you have??)

>It's not putting out any sort of significant heat! By design, the heat is directly vented. If this fireplace sits in a large room you will fill nothing. This fireplace is not a room heater, rather decor. But let it run for few hours, it should warm your old masonry fireplace, and may be you will feel something.

Three years ago , before the pandemic, I wanted to do gas logs. . Although my gas meter was about 6 feet away from the chimney stack, the cost to bring the gas piping to the chimney was going to be $1500, and yes, it did not include the cost of the logs or the installation or permit. So I didn't.

I think an easier and more direct route would be to remove the gas valve and try to identify it instead.