It s probably just a matter of finding a switch with a low enough on resistance.
Gas fireplace switch wiring.
A wiring diagram is a simplified traditional pictorial representation of an electric circuit.
Crimp the connectors securely to the wires.
Slide the spade connectors onto the spade terminals on the back of the switch in the control panel or connect them directly to the pigtail wires if installed.
The thermopile in your fireplace puts out millivolts nothing near the 120v the light switch was designed for.
All the switch is doing is making or breaking a connection it is not wired up to standard 110 volts.
Assortment of gas fireplace wiring diagram.
Slide the provided spade connectors onto the stripped ends of the wires in the gas fireplace control panel.
On the gas fireplaces i have installed the on off switch was a millivolt contact.
A generic low voltage switch from a electronics store or ripped out of a toy would probably do it for example a 12v spst.
If the fireplace is a 120v control then in most cases you will need a neutral.