Gents,
There's one very important, very nasty, and very depressing fact to keep in mind during this discussion about a WW2 chemical exchange; Nazi Germany had nerve agents and the Allies were completely oblivious to the fact.
Germany developed nerve agents like sarin and tabun before WW2 and produced kilotons of them during the war. IIRC, "weaponized" nerve agents were available by early 1942.
Not only were the Allies completely unaware of Germany's nerve agent research and production projects, they were also completely unaware of the possibility of nerve agents. This means that, unlike with other chemical agents, there is no proper masks, no protective clothing, no atropine injectors, and no real defenses against a weapon which is not even suspected to exist.
The response to German troops being gassed outside Dunkirk may not be something as "simple" as gas shells fired into the evacuation zone in return. Instead, Germany's ultimate response may be the nerve gas bombings of military bases and/or cities.
IIRC again, Germany believed the Western Allies, at least, had developed nerve agents too. The censoring of British and US chemical trade journals further convinced Germany of this, but the articles censored had to do with insecticide and/or pesticide research not nerve agents.
Bill