The gas valve is one of the main components on a gas water heater. The control valve is what regulates the amount of gas and air that feeds into the water heater. The thermostat for the water heater is also located on the gas control valve. There is an adjustment that allows for you to adjust the temperature of the water heater.
Some gas control valves also contain a Piezo ignitor which makes it easier to relight the pilot if it has gone out.