gas fireplace?

Be careful. Don't know for sure if this is standard, but I've seen a gas fireplace where the gas supply (solenoid valve) and ignition are electrically controlled.

Also, you want to have them operating in a room with adequate window ventilation as it may not be vented to the outdoors. Higher BTU input (than kitchen range) means more exhaust product being generated.

V

ADDENDUM:

Regarding a generator. A simple generator would likely have no problem operating a fireplace as described above, as it is a simple on/off circuit.

If you are thinking down the road of the possibility of having a gas furnace, then a run-of-the-mill unit could fail to operate it properly. In addition to on/off control, most new(er) furnaces have electronic control boards. Less expensive generators may not produce a 'pure' sine-wave output, such as gotten from the home's power outlets. In some cases, they have been known to damage the control boards.

V

I've had a few gas fireplaces that operate without power - I think that is pretty standard now. Also all the newer ones have been direct vent which have zero ventilation issues. The igniters have all been battery operated.

I think it would be easier to have a generator than come up with a system that doesn't use electricity. I've seen one but it was old and used very large ductwork to carry the heat around the house.