How to find gain for this device? I am doing a project with Arduino to find CO2 in an environment. For this purpose, I bought a breakout board with an MG811 sensor on it. I bought it from eBay and it came with no datasheet. I searched on the internet and found the datasheet for the MG811.

With the help of the datasheet, I also found the formula to convert the input voltage to PPM. But later I realized that the device also has an amplifier on it, which is giving me higher voltages. I searched thoroughly and found the original device on Taobao.

Can someone please tell me how can I determine the gain so that I can convert the values I read to original values?

One more question. On the given web link, I also found that this device comes with a pin called TCM and with Google translate, I found that it shows temperature compensated output. Can you please explain me the use of this pin as well? If it is more advantageous for me, I'll use this instead of the normal analog output pin.
The MG811 spec gives you the range of measurement, but without the values and connections of the surface mount op amp on the board the only way to determine the voltage output for each CO2 concentration is with a Voltmeter and an already calibrated CO2 PPM meter, which you could perhaps borrow from a lab or university.  Once you have the CO2 values and Voltages that correspond, you can build a formula or function to convert the digital values to voltages and then to CO2 concentrations.

The other option is to trace the tracks on the PCB and create a schematic and then post the spec above and the schematic with resistor values included on the electronics Stack Exchange.