Are you looking for a gas log set? Gas log sets can be put inside of an existing fireplace to increase it's efficiency, or to change it's fuel type. Gas log sets come in two variants, vent-less and vented. Vented log sets are much less efficient, but capture more of the feel of a real wood fire, while vent-less log sets are highly efficient and do not require an existing chimney to function, at the cost of the fire looking and sounding artificial.