Building a new home. How to hang the TV above our gas fireplace?

Get the contractors to do for you. Let them know your intentions so the proper materials are used and that the fireplace is vented correctly. They can provide knockouts to run all the wires through the walls too.

Your electrician should be able to do this as he is wiring the house before the sheet rock or wallboard is installed. Talk to him.

I have retrofitted many over the fireplace TV setups. I would cut out an opening in the wall, install 2X4 framing to handle the weight of any size TV. Within the framing I would install two electrical outlet boxes, a plastic electrical box for two 75ohm TV connections, a plastic box for audio cables, and a plastic box for component connections. Then I would cover the framing with the drywall I removed with cutouts for the boxes. New construction is easier than a retrofit. The contractor can provide the proper framing to mount the TV and the different electrical boxes for the connections. The electrician can advise you and them on what you need.

No problem. My son has his tv above the fireplace. He had everything installed when he built his house. The only thing is when someone tall stands in front of the fireplace to get warm, the tv or part of the tv is blocked.

They have kits you can purchase to hang tv's on the wall. Then search on the Internet for "How to hang tv on top of the fireplace."