I’ve been thinking of making artillery shells an actual unit, which you must transport/supply to your armies before the artillery can fire.
One type of shell would indeed be poison gas.
I don’t like techs that don’t have a physical unit, but gas as a tech would add a “yellow” shell type to the standard “red” HE shells and the “black” smoke units. So you choose which type of shell your Heavy Artillery fires in a combat round.
An entirely different approach is to have tactical event cards: the decks will have a certain number of gas attack cards, and a number of gas masks cards to counter these.
So what effects do the various shells have?
HE/SHRAPNEL is used principally to physically destroy defences, particularly barbed wire. It will also of course destroy units, though less effective against entrenched positions.
SMOKE will cancel out defensive advantages such as trenches; essentially it allows the attackers to reach enemy positions before they’re seen. Same effect as FOG (q.v. Weather).
Effects of GAS are more difficult to determine, I would say more to do with disabling infantry units rather than killing them, especially if the target units have gas masks. Effected units would be unable to fire in the battle, but would most likely recover afterwards, with say a 1/6 chance of dying from the effects.
http://en.wikipedia.org/wiki/Poison_gas_in_World_War_I
Too complex? This is Axis and Allies, not Risk.