As its name implies, the gas control valve in a furnace controls the flow of gas to the furnace. However, the design of control valves varies considerably between different models of furnace depending on the ignition system used by a particular furnace.
Older furnaces may have multiple gas control valves for controlling gas flow to the pilot light and the furnace itself. In many newer designs, the pilot light is responsible for opening the main gas valve as using a flame sensor or thermocouple system. Furnaces that use direct electric ignition do not need pilot lights at all and only have a control valve for the main gas supply.