Many gas fires and stoves use a conventional flexible flue liner to remove the combustion gasses from the appliance. This type of flue is typically dropped down an existing chimney.
A gas fire that uses a conventional flue may require a room air vent to provide sufficient combustion air. If you do not have a chimney, then a balanced or twin wall flue can be used.
Some glass fronted gas fires and stoves use a balanced flue to introduce combustion air and expel combustion gasses. This is the most efficient way to flue a gas fire or stove.
A balanced flue can be very short and will typically exit behind the fire through an exterior wall of the room. With a balanced flue there is no need for a room air vent.
If you don't have a chimney, you can still have a conventional gas fire installed using a rigid twin wall flue system. This can be fitted through the building or attached to an outside wall.
Twin wall systems are insulated to ensure that they draw well in the coldest Winter months. Twin wall flue systems are more expensive to instal than conventional or balanced flues.