The so-called "Paris Gun" was largely a psychological weapon the terrorize and demoralize the population of the French capital. As a military weapon, it was fairly useless. The Paris Gun could only fire AT MOST once every 15 minutes, and the barrel tended to need to be changed about every 20-30 shots, an operation which took several hours. Additionally, the Paris Gun only fired a 106 kilogram shell, which is actually similar than the shells used by the 21 cm Moerser line of heavy howitzers, upon which the mechanisms and shells of the Paris Gun were based. The result is that although the Paris Gun could reach Paris, it delivered, in comparison to it's size and expense, a small payload, inexactly placed, at long intervals. Using a chemical weapon would have only served to enrage the French -- since even the Germans had the fig leaf that they only used chemical weapons against enemy troops.

On a slightly different note, I'm not certain that it is physically possible to use a chemical-filled shell with the 21 cm Moerser -- the heat and acceleration from the firing might breakdown the chemical agent or cause a rupture in the containment vessel.