Gas heaters warm up the area through the combustion of a gas, most often a propane-butane mix. The warmth coming off this is big, making this type of heater the right choice for heating garages, work places and big halls.
Depending on the size of the area you choose a gas heater with the appropriate amount of power.
Important to pay attention to is airing the room in which the gas heater is operating. While burning gas, carbon monoxide is released and oxygen is used up. Both of these effects are not desirable for the health, so an appropriate amount of fresh air is of importance.