Sorry I am confused I assume the first pic is what you want. In most cases the hearth can be removed with a gas FP as for the trim ask the inspector where you live . Usually the FP store knows the rules so maybe there are no specific rules . Gas FPs are usually not a big deal

Yes, Patricia, you're confused. The OP has a full-masonry fireplace that had a wood-burning insert which has been removed. They simply want to install gas logs in a masonry fireplace. I did that in my former home. The clearances they are inquiring about for a mantle and surround which they depicted in white. It's not clear what building code applies to the OP's installation, but the 2018 IRC, which forms the core of most statewide building codes, specifies them in section R1001.11 Here's the pertinent specification from that section of the code:

(4) "Exposed combustible materials or trim is permitted to be placed directly on the masonry fireplace front surrounding the fireplace opening providing such combustible materials are not placed within 6 inches (152 mm) of a fireplace opening. Combustible material within 12 inches of a fireplace opening shall not project more than 1/8 inch (3 mm) for each 1 inch (25 mm) distance from such an opening."

Whatever you decide, Keep a fire extinguisher handy for at least the first five years.

I second what Charles Ross said. I have been selling Gas Logs for over 35 years and I can tell you that the clearance specifications remain the same for the fireplace whether you burn wood or gas logs. Charles has provided the typical accepted clearances for a masonry fireplace, which is what you have. Prefabricated zero clearance fireplaces have their own clearances, which you would get from the owners manual. Unlike typical gas logs, a gas insert may have it's own clearances, which will be provided in the installation manual.