This topic was not revisited for some time, I am sorry if it was discussed elsewhere. Here’s a game mechanic suggestion to keep it simple:
1. No need for a new unit. Decide whether an infantry or artillery will be capable of gas attack (or both can be upgraded). Historically, hand grenades, 90 kg cannisters carried by soldiers, mortars, artillery shells were used. For the game, choose one of two basic land units. The new upgraded unit will be more expensive, and could be marked using something.
2. The new effect concept is “force retreat”: The idea is not to kill, but to disrupt the trench by forcing a number of units back one territory and thus to weaken the stack. In AA1914, large stacks of infantry are at the center of the game mechanic, so the gas attack should be about this. A gas infantry rolls to hit normally, no change necessary. Upon hit, 2 defending units (could be 1) are forced to retreat from the contested zone, to any neighboring zone of choice containing own units. (That zone might also be contested.) Retreating units do not have defense rolls and they have to spend the next turn in a hospital or triage, so they cannot move. They can defend the zone they are healing in.
3. Gas infantry comes with protection, so they are immune to gas when defending.
4. A counter tech that can be used is of course gas masks, if tech rolling is used. They can be universally applied if developed, but then producing gas infantry would be useless. A second upgrade can be used to produce masked infantry (more expensive than infantry, less than gas infantry?) Gas masks would complicate matters I guess.