Depending on the BTU output of the gas fireplace, the heights can differ. Refer to your gas fireplace manual, and it should be stated in there. For ours, an 8" wood mantel need a minimum of 22" above the top of the heat outlet. However, a noncombustible mantel can be installed anywhere above the heat outlet. You should be able to find your manual by doing a quick google search of your fireplace.

Look at the installation instructions for your particular fireplace. If in doubt, contact your local Building Official.

Believe it or not, the only code for a gas fireplaces is to "read and follow the manufacturer's instructions".

As mentioned, the requirements can vary widely.

Having said that, placing a mantel at the minimum height required almost never looks good. It will take a little more figuring to establish what will meet the requirements and look nice. Do you have a photo you can post?

this is a space for the fireplace. there will be a TV above the mantle.