If you already have a prefab or a masonry fireplace then a gas insert may be for you. Enjoy real flames without the hassle of solid fuels. There are basically two types of gas inserts. Direct vent and ventless inserts.
The first type are direct vent units which can be vented through the existing chimney. The air inside your house and your neighborhood stays clean. Direct-vent units are safest. They use the outside air to keep the flames burning. These also exhaust combustion gases and water vapor that is released by burning gas.
You can also choose one of the vent less inserts that are available. The vent less inserts have a higher efficiency rating as there is no heat to escape up the chimney. But there may be some downsides to this type too. Exhaust fumes and mositure is released while burning and these remain in your house. An oxygen depletion sensor is equipped within the vent less stove. It turns the gas off before carbon monoxide becomes a dangerous level in the room.
One downside to a gas fireplace is that you will spend more on gas or propane than you would spend on wood or wood pellets to produce the same amount of heat.