The house we had in Oregon had a geothermal heat pump. No condenser and fan, heat exchange occurred in a cooling loop drilled down in the ground. For the typical winters there I can probably count on 2 hands the number of times the electric coils had to supplement the geothermal for heat. A very efficient system and the outdoor unit was so quiet you could hardly hear it run.
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Back in 2005, we had an ice storm that took down power lines. We have a gas insert in the fireplace that gaved us some heat. The next night, my wife and I took sleeping bags and along with our dog, went to her office where there was electrical power and heat. My wife’s boss came in with her family. Her boss and her husband found that generators were available just over the Indiana/Michigan boarder. She asked if we would like a generator, so we decided to buy one. They got back about 1:00 a.m. We loaded a generator into our van and stayed in the office. About 4 a.m. our dog woke me up. I decided to try calling our house and was amazed that our answering machine came on. It had to have electric power to operate. What I found out is that my neighborhood is on the same power loop as a WalMart and Lowe’s store near us. Apparently, these stores are on a high priority list to have power restored. Other parts of our region were without power for almost a week.
I have installed a connection to power the gas furnace controls and blower from the generator, but I have never had to connect the generator.
Clearly this varies by location. We have lived in the Duluth, MN area for 20+ years, we have one of the coldest climates in the lower 48 and receive heavy snowfall (not yet this year) with a lot of tree cover. I don’t recall one time in 20 years that we had a power outage that affected the ability to heat our home - it’s just not something I’m concerned with up here. Sure, there is an outage or two that is restricted to certain areas but the power company is top notch and generally restores power quickly. I would be more concerned with losing power in the summer in FL or AZ, both places we have lived as well. IMO, it’s easy to keep warm (and your food cold) in the event of a power outage in the winter vs staying cool in the summer.
For that latitude, I think geothermal is by far the best system. It keeps the heat pump in the upper range of efficiency since the groundwater doesn’t get below about 60 degrees. And then AC is almost free since you are pumping house heat directly into the cooler ground
I investigated one for my Ohio home as did the guy who bought it. I was ready to do it until we had to move for my job. The buyer didn’t want to spend the high install cost.
I bought a standalone heat pump for the pool about 12 years ago. It has a scroll pump so very efficient and reliable. First year, for grins, I ran it all the way to Halloween with temps dipping into the 40s at night and 60s during the day. Steam coming off the water 
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Some of my neighbors have them for when the electricity goes out. I can hear their motors running. Presumably they are small gasoline generators mostly to keep the freezer and fridge working.
My gas cooktop uses electricity to make the spark which ignites the burner. But it still works without electricity in a power outage, just have to light the burner with a match. I’m aware the newer water heaters don’t have a pilot, but hard to understand how that makes them more efficient. The burning pilot heat isn’t wasted, it still heats the tank. Most newer furnaces require electricity to run the blower, but no blower in gravity furnaces. I wonder if newer gravity furnaces are pilot-less? If so, that would defeat their advantage of still working when the power is out.
Colder areas, gravity furnaces less common. My family home in Colorado, late 50’s construction, had the normal blower-equipped natural gas furnace, and my job was to keep the bearings lubed. But one of my Colorado relative’s home, 1930’s construction, used a gravity floor furnace. That floor furnace provided very comfortable heating, but the heat didn’t get into all the corners of the house unless some fans were turned on.
It wouldn’t matter. The thermostat that turns the furnace on needs electricity to work. The valve that releases the gas burned by the furnace is also powered by electricity. Whether pilotless or piloted, you still need electricity if you want heat.
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I was confused when you called them ‘gravity furnaces’. I’m familiar with the old fashioned ones that you light with a match and they glow. Not the safest thing in the world, and I wonder about the fumes they generate, or leak while turned off. Newer ones would need electricity, as @Mustangman noted.
My current gravity furnace has no electrical connection. The electric current needed to turn the gas valve on is generated by the pilot flame heating the thermocouple. Same as all the natural gas water heaters I’ve ever had.
I was confused by that terminology too. As far as I can tell it refers to the fact that heated air is less dense and therefore rises until it cools, then it becomes more dense and falls. That is the force that moves the heated air to and fro, said to be powered by gravity.
I would expect they were pretty common in warmer parts of Texas, gulf coastal areas, in homes built before 1960.
The old house my parents owned had a coal fired forced air furnace. My dad had a Hanley Brown gas conversion burner installed. There was a lever on the gas solenoid that one could flip that would allow the burner to come on if the power was off. One had to be careful that the bonnet on the furnace would not overheat and set the floor on fire.
In the addition of houses built just after WW 2 where I lived from 1977 to 1989 had gravity gas furnaces. My house had the original gas gravity furnace replaced by a forced air furnace before I bought the house. However, my neighbor still had the original gas furnace and the burner could be activated when the electricity was off by flipping a lever as did the gas conversion burner on the house where I grew up.
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The gravity furnace is obsolete. This type of furnace is not a factor is California’s ban on natural gas systems in future new home construction.
How do you adjust the temperature?
The thermopile supplies 750mV to the thermostat. These are used in gas fireplaces as well.
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OK, I get it now…
The link you posted helped me find this;
Notice I said modern furnaces. Of course there are much older systems that had no modern safeguards or electronic controls. Modern furnaces will not be able to do flame detection, operate the control board with all of its interlock protections or any relays on it to open the main gas solenoid without power. My propane oven also has electronic controls with similar limitations. It uses a sparker to ignite the top burners but the oven uses a heated element and a sophisticated electronic control to determine if the heating element is up to temperature before it will open the gas valve. Then it does flame detection to ensure it lit properly. That ignition heater uses a lot of current to get orange hot.
My brother installed a propane heater in his fireplace that does not require external power to run. But it is limited without power because the convection fan doesn’t run. He, like most people around here that rely on heat in outages, has a portable gas generator. Many homes have standby generators in my neck of the woods.
Just an interesting side note- some furnaces use ionization rectification to detect flame. The burning gas flame is used to conduct ground from a flame rod to earth ground. So if you disconnect the furnace and power it directly from a portable gas generator, there is no earth reference and the flame detection will not work.
We have an 8,000 watt generator. It was on sale for $100 more then the same brand (Briggs) 5,000 watt generator. Our water heater is an indirect water heater that is just a zone off the furnace - so no problem there.
I personally know a guy who owns and lives on half an island off the coast of Maine. Totally off the grid. He built a beautiful 4,000+ sq/ft timber frame home. Super insulated. Solar and wind power. Has 2 large propane tanks for heating, cooking and backup generator. Also has a gas refrigerator. And he’s close enough to shore he has good cell coverage. Satellite dish for TV. Last I talked to him he had been living there for a few years. Never had a power issue. He’s now looking at a wave generator (nice to have that kind of money). His 24’ Cabin Cruiser has been converted to run on propane also.
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I grew up in a home with a gravity furnace. It was a converted coal furnace that now runs on gas. It literally took up 1/3rd of the cellar.
Same here, big pipes to distribute the heated water to the radiators, still had an opening on the basement wall where they had delivered the coal. But I think George’s ‘gravity furnace’ is just a gas radiant heater, depending on air convection, not water convection.