Poison Gas is a staple of many dungeons.
There's not much to add in terms of WHAT a poison gas does: it might simply be save-or-die, it might lower an attribute score, it might cause paralysis, etc.
However, I was thinking about one particular style of the delivery of poison gas, which is more centered on how it operates in a game, as opposed to how it operates in terms of effect.
It has to do with the "puzzle," however it's posed (as a puzzle or just a thing to manipulate for right or wrong). Poison gas would in many circumstances be delivered by the combination of two chemicals, right?
Give the players the chance to do this themselves. In general, this would work best if some of the possible combinations are actually beneficial. I'm not going to go into possible beneficial results of an inhaled gas for the same reason that I'm not going to go into negative effects - you can probably generate that list in a few seconds. However, here are a few "scenarios" in which a combination of how the players could cause the release of a gas.
Pouring stuff from a kettle into the open top of an idol (second chemical is in the idol's belly. Picking the right thing to pour would affect which gas you get.
Two bars of a solid can be touched together (possibly with a machine that could allow you to combine different ones). When the solids touch, it causes a chemical reaction where they begin to sublimate into the gas when they touch.
Diverting a channel of one liquid into another channel (mechanisms to allow you to do so).
Opening more than one vent to release two gases that are individually harmless but combine into poison (or something good). I can see a room with several vents, and possibly combinations of 3 gases as well as just 2.
Potion bottles that in addition to containing potions release wafts of harmless gases that, as above, combine if they are in the same room.
Liquid that if poured onto an altar (which is smeared with a solid) causes a chemical reaction with that solid...