When moving or installing a gas line, you will need a plumbing contractor, fuel gas contractor, or mechanical contractor who is licensed and certified for fuel gas.
Which type of contractor depends on what licensing is required in your area and what the contractor is licensed for. As an example, many plumbing contractors are licensed to install, move, or repair gas lines, however, it is possible that not all plumbing contractors in the same area will be licensed for performing that work.
In many areas, mechanical contractors (contractor who install heating and cooling systems) also carry licenses to install, move or repair fuel gas lines.
Now, when moving the gas fireplace itself, you will want to have the installation instructions for your particular fireplace available so you can make sure the installation of the gas fireplace in its new location meets all of the installation requirements specified for that particular gas fireplace.
Most installation instructions for gas fireplaces can be found on the internet of the various manufacturer's websites, and, if you cannot find the installation instructions on their web site, contact them and they should be able to send you a copy of the installation instructions.
Making sure that the installation of the gas fireplace complies with the installation instructions is VERY IMPORTANT, otherwise there is a potential great risk of fire.
Finally, you would need to make sure that there is NO DAMAGE to your existing gas fireplace. This is also VERY IMPORTANT. I recommend you have your gas fireplace inspected by a gas fireplace contractor after it is removed from its current location and before it is installed in its new location, if there is any damage which is repairable, have that fireplace contractor repair your existing gas fireplace, if the damage is not repairable - DO NOT RE-USE YOUR OLD GAS FIREPLACE!
One question for you: Is your gas fireplace vented or ventless?
A "vented" gas fireplace has a vent (metal chimney) which goes up and "vents" to the outdoors through which all products of combustion exhaust to (are vented to) outdoors.
A "ventless" gas fireplace is has no vent to the outdoors, all products of combustion are 'vented' into the living space. These types of gas fireplaces come with strong warnings not to operate the gas fireplace without opening windows and doors for ventilation.
Jerry Peck - CodeMan
Construction and Litigation Consultant
Construction Litigation Consultants, LLC.