You need to know what logset/ burner you are installing. The location of the gas inlet will dictate what is possible. Since most valves are located on the right hand side (because most people are right handed), a gas line entering the right will tend to push the logset past center. You don't want some jumbled up mess of spaghettic bunched up at the valve. Usually, by entering a factory built fireplace knockout from the left or a masonry from the ash pit as JTP described is the best approach. You can easily route you line while keeping it centered, towards the rear with adequate secondary airflow in the sides.
Do you know if this fireplace and room are suitable for gas logs?
Are these vented or ventfree logs?
If ventfree, are they allowed to be installed here and do they meet the sizing guides and have adequate ventilation?
Has anyone performed a Level II inspection on this fireplace and chimney?
Lot's to know and consider before charging off into this work. I suggest you get some training and hearth credentials first. Start off by talking with your local hearth dealers.
You should consult a PRO for a ventfree. There are so many little things that need to be considered. Is the cost saved on labor by DIY worth you and your families lives? Also, make sure you get a good CO detector.