Gas heaters still require electricity for basic management.
I'd imagine heaters have batteries for specifically this case. If they don't, it's frankly a miracle if we don't lose anyone, and someone needs to get prosecuted.
It is not feasible to run a gas furnace off a battery (short of a "whole house" battery like a Powerball). It's more than just basic management. You need to run either a blower or a pump to distribute the heat throughout the house. You'd need a substantial battery to run a 1/3 HP motor for any length of time.
A gas boiler furnace with steam radiators typically only requires power for the thermostat and solenoid valve, which can indeed be powered by a small battery. This is assuming you have a pilot light, which many older furnaces do.
Residential steam boilers are pretty rare these days. Generally hot water heat uses circulation pumps (which draw less than a forced air blower, but still have a non-trivial continuous current draw)
Furnaces don’t come with batteries because a 12v 100Ah battery that costs $180 will run a 1/3rd HP (700w) fan for 2 hours. To sustain the fan for 3 days, you’d need 36 batteries. Lead-acid batteries have a limited lifespan. You would also need a transfer switch and enclosures and at that point you may as well buy a UPS or a natural gas generator to provide backup power.
One thing the NEC 2020 update allowed is bi-directional power between an EV and its charger, you can set up an EV to provide backup power your house, with a transfer switch and everything.
Radiant heating systems with boilers require electricity for the pump, even if they’re gas fired. Electric resistive heating needs electricity. Forced air heating needs electricity to run the fan. All heating systems require electric power, even if the heat is generated by burning natural gas.
We're talking about forced air furnaces here, I think. Something has to force the air. A big blower motor that takes hundreds of watts.
Forced air heat is not great in terms of comfort, but in Texas you already have all the ductwork and the blower motor and so on for AC, so it's simplest just to add a gas burner to that.