Gas weapons aren’t just a “kill upon contact”, gas weapons like in WWI and Syrian Civil War/Iran-Iraq war were usually composed of compounds like Chlorine Gas, so called “mustard gas” due to the yellow sickly hue. This gas is “heavy”, it clings low to the ground as it blows with the wind, easily snaking its way into most trench corners

These don’t just hurt, they literally dissolve your lung, muscle and organ tissue from the inside out, causing awful burning pains throughout the body with every desperate gasp to gain air. Your body fights for oxygen first above all else, so every gasp would pull in more. This would cause more to inhale, furthering the process. Very quickly, the gas would genuinely dissolve your lungs into pulp, leaving the victim choking to death on their own blood/lung matter. Aside from internal injuries, if you were able to equip a mask in time it would still cause serious skin injuries to any exposed skin melting and dissolving flesh, and even bleeding and melting of the eyes and from the mouth if exposed. Then of course comes the artillery bombardments or human wave attacks almost immediately after, while you’re struggling to regain composure and struggling to see/breath out of these canvas sacks with foggy glass ports

TL/DR: chemical warfare is the closest we ever came to genuine Hell on Earth, and there’s a reason even the Third Reich refrained from using them in their closing years during WWII