There's this thing called a fan that's inside your furnace. If the electricity goes out - you're gonna die of cold anyway because there's no way to move the heat from the furnace through your ductwork.

Gas valve and igniter run on low voltage, I think the combustion blower runs on 120 VAC, the main blower that distributes the heat to the rest of the house is definitely 120 VAC. A small gas generator that can provide maybe a kW would probably be the most straightforward backup.

Still won't work. If your home is sufficiently air-tight so that you won't die of cold with that paltry amount of gas, you're going to die from the carbon monoxide from your stove.

The propane heater from Home Depot either has the same problem or it uses electricity for the air induction system.

If your goal is to be prepared, a generator and a heat pump will last longer, it will be cheaper for you and society in the long run, it will have more utility, and we have far more gasoline stockpiled and ready for emergencies than we have natural gas.

How much is a generator though?