It's true that most gas heaters need electricity, but there are exceptions.

The more modestly priced ones use a piezoelectric lighter and forced air fan, which I assume cuts costs by requiring a less robust heat exchanger. Line power is mandatory for operation.

But there are units (premium cost) that use an old-fashioned pilot light and a heavy duty exchanger. They have forced air fans also, but do not need them to operate. A couple of these units kept my parents' house from freezing after a fire knocked out the main panel. They work.