DO NOT ALTER THE WIRING TO THE GAS VALVE!!!! This uses a modulating valve...not simple stuff. If you don't know what you're talking about...don't offer assistance. There are a million threads on here about adding thermostats to these units. A few I have gone into great detail on. Basically every method you can think of is bad for the heater.

If you are just going for heat only just connect the t_stat at these setting red wire R white wire to W green wire to G

Now the heater you can go to the gas valve or if it has a circuit board go to Red to R and white to W Green to G. Green is optional you just need two wires hooked up. You will see on top of the gas valve three connections the ones marked th are the ones you need.