The gas pedal is an accepted slang term / drivers parlance for what is more accurately called the automobile accelerator pedal. It is informally called the "gas pedal" because in vehicles with internal combustion engines, which happen to be the majority of vehicles currently, depressing this pedal will cause more fuel (in the form of gasoline or some other inflamable) to enter the vehicles' combustion chamber and thus make the engine run faster in turn accelerating the automobile. Gas == Gasoline.
Of course, this implies that in the part of world it is being used in that the name for "Gasoline" is, well, Gasoline. Usually that means North America, but in places where it goes by another name, more often then naught as "Petrol", the Accelerator Pedal will be know informally as just the "Accelerator"; places like Britain, Australia, New Zealand, etc.
Special thanks to Fruan.