Very interesting discussion (made me sign up!), especially since I have a passion for chemistry myself
You have brought up chlorine due to the green colour and quick (yet not
instant) lethality, and abundance. That would fit.
However, the Sligs are unaffected by it - provable in both games. It could be argued that their masks work as gas masks.
But... In the end of Oddysey, you can notice (by dropping into the board room the very last second) that Glukkons are completely immune to the gas as well.
Logic-wise, the gas would probably have to be a very species-specific (in fact,
Mudokon-specific) poison.
It is possible however that the devs based their fictional toxic gas on chlorine.