D&D has a far more advanced alchemy trade than traditional fantasy than our world did, and some gasses are pretty easy to create.
I mean shit, they start with Chlorine gas which anyone could make, and then someone doing a study attempts to create more and more powerful and effect weapons.
Shit, it could be a weapon of the BBEG. What would be more effective for a Necromancer or Lich to quickly kill thousands or even millions of people, leaving corpses pretty much untouched to serve as zombies or something better?
Again, D&D already has magical nerve agents, see cloudkill. Nothing to suggest more mundane versions are impossible to create.