While it was not nearly as big a factor as it was in the first world war, all major axis and allied powers still had massive gas stockpiles during the war. It could really add an interest element to the gameplay, if done correctly.

Well, since it was always possible that poison gas could have been used (gas masks were used in Britain over that worry) then it could be put into a scenario, but if so, it ought to cause a diplo hit with all nations as would a nuke.