A generic low voltage switch from a electronics store or ripped out of a toy would probably do it for example a 12v spst.
Gas fireplace switch wiring.
If this is the case there will be no power to run the smart switch unless it is battery powered.
The thermopile in your fireplace puts out millivolts nothing near the 120v the light switch was designed for.
It s probably just a matter of finding a switch with a low enough on resistance.
Crimp the connectors securely to the wires.
Slide the provided spade connectors onto the stripped ends of the wires in the gas fireplace control panel.
It shows the parts of the circuit as streamlined shapes and also the power and signal links in between the devices.
If the fireplace is a 120v control then in most cases you will need a neutral.
All the switch is doing is making or breaking a connection it is not wired up to standard 110 volts.
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.
On the gas fireplaces i have installed the on off switch was a millivolt contact.