If your gas furnace is controlled by a 24VAC system panel, then the Simple thermostat is likely compatible with your system. However, many homes have independently controlled furnaces. If your existing thermostat is directly wired to the furnace, you'll need to evaluate the type of furnace system you have.
One of the most common types of controls for indoor gas furnaces is the millivolt gas valve, with TH, TP, and TH/TP terminals, as shown below:
If you have a millivolt gas valve, the Simple thermostat is not compatible with your system, as it is unable to supply sufficient power to thermostat.
If you have any questions about your particular system, feel free to contact us at firstname.lastname@example.org.