My gas fireplace has a valve mounted flush with the floor. You use a special key-like wrench to turn it on.

Even with the valve barely open you have maybe 3 to 5 seconds to get it lit in a normal fashion. Heh mess around too long and you’re in for a big woosh and plenty of singed arm hair.

My gas fireplace still requires electricity to run. I know I can start it with a match if needed (and it has a pilot of course) but I'm not 100% sure if I can manually override the gas valve to open. It opens when I flip an electric switch.

Does it? Mine opens when I flip an electric switch, but it's self-powered via a thermopile on the pilot-- no AC connection whatsoever.

Some other models (from the same manufacturer as mine) don't have a standing pilot, but either use a battery as the primary ignition source, or use AC power but take batteries as a backup.

I've seen both styles. If it is a regular switch in the wall it uses mains to open the valve. If it has a switch on it, or a thermostat on the wall it probably is thermocouple powered. Though there are exceptions both ways.

Even with the switch on the wall nothing is certain. See my sibling post to parent. We had a wall thermostat but all it did was send a trigger current. If the thermo pile wasn't generating enough current nothing would happen :)

Sooo many different systems and many are just not well thought out. Especially in climates where it usually doesn't matter.

Here in Quebec it does matter so the one we have works in all conditions with various levels of manual intervention. We don't even have the one with a battery operated fan but even that exists.

Try it. I was confused the first time I used ours when the power was out. That switch still worked.