Well, but in this case they're just homonyms, like how the word "bank" can refer to the edge of a river or a financial institution. The gas in your car is short for "gasoline"; you wouldn't call oxygen a type of gasoline, it's a gas.
Actually, "bank" isn't just homonyms; both senses derive from circa-Proto-Germanic benc/bank: a bench or other raised area (either a bank counter or the raised ground adjacent to a riverbed). (And yes, this etymology is also shared with "bench".)
