The main gas used in WWI was mustard gas. This gas was very different than any other gas used to harm people. It would work on any open skin, your eyes, mouth, or even your ears. This was more effective than the other gases that had to be inahaled. Gas masks could provide some protection but not enough for most people.
I do think Remarque gives an accurate description of how the gas effected the troop in WWI. He talks about how it gave excrutiating blisters on open skin not just when you inhaled it.