When the power goes off
When the power goes off, so do furnaces, fireplace inserts and some ovens. During a power outage, a natural gas furnace won't work because the blower motors need electricity to operate.
Most furnaces have a 24-volt safety valve that closes the gas valve when the power goes out. Although the pilot light may keep burning, the valve will not open until power is restored. At that time, it will open automatically.
Fireplace inserts also need electricity to function. Depending on the year and model, your oven may or may not need electricity. In any event, never use an oven to heat a room.
Most gas fireplaces, water heaters, ranges and log lighters will operate when the electricity goes out because they don't require electricity. So they'll continue to work during a power outage. Gas logs and log lighters will be able to be turned on by hand.
The top burners on your range also will work. But range tops with electronic ignitions will need to be lit by match. Light the match first, hold it next to a front burner, and turn on the burner very slowly and with caution. When in doubt, consult your owner's manual for instructions.