How to find the right thermostat for my gas fireplace?
The information you supplied in your question seems contradictory. A millivolt thermostat is specifically different then a 24vt thermostat.

I would suggest you look for and confirm the existence of the referenced "low voltage transformer" and confirm whether or not it is part of the thermostat circuit loop (and confirm its voltage). If it is there then the 24vt thermostat is what you need.

Alternatively a "Thermopile" as referenced is a very low voltage electricity generator heated by the pilot. If that is the power source for the gas control valve and thermostat then you need a millivolt thermostat.