All gases have different properties. Some gases certainly do emit infrared energy. However, they normally only emit in a very narrow spectral band. Thus seeing a gas is certainly possibly by using a camera adapted with a suitable spectral filter. Seeing the gases is not the same as measuring its temperature, and there are additional complexities when it comes to measuring a gas temperature. A gas is not a solid, and it has a varying density, thickness etc. Because the context is changing, so too will the apparent emissivity. So measuring the temperature of a gas is one of those things that sounds very easy, but turns out to be virtually impossible in most cases.
OGI thermography uses a camera spectrally adapted for visualisation of a gas. They are not really designed to measure a gas temperature.