I assume this is a wood burning fireplace and you want to convert to gas burning. The conversion requires routing of the gas supply line, installation of the gas burner and ceramic logs. More efficiency may be possible with modifications to the flue system, but it isn't necessarily desirable to make it vent-less due to moisture buildup from gas combustion. Consult with a wood stove or fireplace vendor to find an appropriate set and to obtain recommendations for an insert or installation the meets your needs. There may be code requirements and safety concerns that we can't address without knowing more about the setup. I believe its well worth a professional installation for the peace of mind, and its usually a fair deal to have the fireplace seller install the unit and route the gas line.