First TURN THE GAS OFF BEFORE YOU DO ANYTHING!
Page 29 [Manual link below] has the wiring diagram. I would check the voltage output of the AC adapter, or if installed, the voltage from the battery backup. Then measure across the ON/OFF switch and see if there is DC voltage. If there isn't, then you simply connect a Zigbee or Z-Wave door/window sensor across the two wires going to the ON/OFF switch.
If there is voltage, get a DC relay with a coil rated for that same voltage and connect it to the side of the ON/OFF switch were you measure the voltage so when the switch is ON, the relay will close and when the switch is OFF, the relay will open. Connect the Zigbee or Z-Wave door/window sensor to the no-voltage contacts (e.g. Dry Contacts) of the relay. Result would then be the relay turning on will close its dry contacts and that will in turn trigger the Zigbee or Z-Wave door/ window sensor, thus telling you in HE the state of the fireplace.
