The gas fireplace unit will have installation instructions. You must thoroughly read these instructions as each unit will have different rules about haw they are to be installed. For instance many of these units require a special cement board be placed around the unit and particular setbacks or standoffs must be observed when framing the unit into the wall. Several units I have installed required metal studs as well. In terms of the gas line the unit must have a gas shutoff in the same room as the appliance. On the outside of the home you must observe the minimum clearances to windows and doors and the ground. There are minimums in the code but the manufacturer instructions and minimums take precedence as they are the testing facility and they set the minimums for their unit. These minimums are usually greater than the code specifies as the minimums.