Gas log systems are meant to be installed in a current up to code and in good working condition wood burning fireplace.  Two different types of fireplaces.  The first being a masonry system either factory built or hand built.  The second unit is called a Zero-Clearance wood fireplace.  This second unit can be identified by a license plate on the upper part of the opening.  Most cities and counties require the gas line to have a key valve or gas cock within three feet of the opening but not in the opening.  Depending on your building department there are many different requirements.  One would be a glass door or auto damper system.  Some departments also required screens over the opening.  In almost all cases the local officials require the clamping open or removal of any manual dampers.  Combustion air inlets may need to be added to bring the fireplace up to code.  Gas logs are usually the asthetic choice and should not be made as a heating appliance.  Gas logs look the best as far as flame pattern and burn but again the heat usually goes up the chimney.