Why is it called a gas gauge?
I guess it's derived from the American term for vehicle fuel being referred to as gas. Many countries refer to it as fuel or petrol. The Wikipedia article Fuel gauge starts off with the following:


  A fuel gauge (or gas gauge) is an instrument used to indicate the level of fuel contained in a tank


So I guess from that point of view it makes sense, when a car runs out of gas / fuel it will no longer operate and the same applies to a battery.