It would depend on how early in the war tho happens.
If it happened in 1980 or early '81, Iran would not have transformed into a dictatorial mullacraty that picks up little boys from the street to turn them into 'holy warriors' and have then walk through minefields with nothing but a Koran for protection. In fact between the revolution and the war and even in the first year of the war they were very concerned about their public image and at the same time they were keeping the personal of the American embassy jodtave, they were also trying to promote heir mountains as a ski resort and American journalists covering the hostage crisis were generally given the most heartfelt welcome possible. If Irak would have resorted to using mustard gas against Iranian cities at that time, the public opinion outside of the US would pretty much directly shift towards sympathy for Iran. Even inside the US voices will rise... All Iran has to do is do nothing and show the world that they are the real victims here.
If Irak only resorts to gassing Iranian cities after 1984, no one will bother anymore. Both Iran and Irak are at that time so villainized that may be Israel might get nervous, but every other nation would just shrug and think 'Well, good thing they only use those weapons to clobber each other'