It would make far more sense to just pound them all day with standard 155 HE rounds. It should scare you wayyyy more than a gas round.
If a 155 gas round hits close to your position, you even have a sporting chance to run for your life and get away, especially if the wind gives you an edge. But if an HE round hits, there -ain’t- no running for your life. It got you, or it didn’t.
Gas is scary in both non-logical and perfectly real ways, particularly to a civilian population. Gas can penetrate bomb shelters and trenches, particularly the kind that civilians would make in a hurry. It can be dispersed with weapons that have little “bang”, so it can be widely faked with small expenditure of real gas shells.
Some real gas mixed in with a scattered bombardment across wide urban areas can cause a massive civilian panic.
Remember that Assads government hates its people. It works by scaring them.
That is why you mix smoke, high explosives, and chemical weapon rounds together in a fire mission. The effect of high explosive rounds mixed with smoke and the occasional chemical rounds will not only cause confusion but will also enhance the chemical rounds because of the element of sunrise and the unknown.
Many of the indigenous militiamen are returning HOME to their families in lulls in fighting.
Militiamen become completely ineffective if they are responsible for transporting parents, grandparents, inlaws, wife and kids in a desperate race to see refuge across the border, or even across ten miles of urban battlefield.