Some do, some don't. More and more appliances require electricity these days, because they use digital electronics. The designers do not care what happens without electricity, because almost all their customers have electricity almost all the time.Gas water heaters don't require electricity, Wally.
Seriously, whatever else you do, do not hook up a car battery to a gas water heater. That would combine uncontrolled sparks with natural gas, and quite probably cause a lot of damage.