What is a poison gas? Technically, you'd speak about "Airborne toxic agent" or "airborne chemical weapon", as gas is an aggregate and most chemical agents that are not gas-shaped are powders or droplets carried by air. Now, what is there?
One of the most simple airborne chemicals is smoke. Thick smoke from burning wet wood or unpacked, loose gunpowder can be used to block sight and reduce the ability to fight in it, demanding the use of wet-cloth 'gasmasks' to be able to fight for more than a short time in it.
To increase the effectiveness, adding more sulphur into the mix would be a great way: the produced SO, SO2 and SO3 turn into acid in the lungs of victims. Sulphur was used in such capacity at least as a secondary effect historically when using flaming siege ammunition made from bags of tar and sulphur. Such ammunition also is harder to douse with water, as it burns hotter.
Another agent that, when burned, creates very bright light as well as thick and caustic fumes are derived from phosphor. White Phosphor was first isolated in 1669, which is pre-industrial post-medieval. It ise feasable that it could be isolated earlier, as the isolation process taken was simply cooking urine.
In the tunnel battles under Vienna during the siege of the Ottomans in 1529, smoke was used en-masse to try to drive out siege and countersiege tunnling by removing visibility, displacing oxygen and making it hell down there.
Arsine (AsH3) is a flameable and pyrophoric... and highly toxic. It could be generated on the battlefield by pouring a strong acid over a dry powder mixed of Arsene-III-oxide (natural occuring) and Zink powder (refinement of Zinc is known since antics). The main problem with the use would be the high flameability, though in tunnels this would be actually a huge benefit: if someone brings a torch to light it, they also blow out the tunnel, killing combatants.
Similarilay, Prussic Acid, aka Hydrogen Cyanide, can be created by letting ammonia gas blow through glowing coal. Ammonia Gas can be generated by mixing ammonia salts with quicklime.
The Ammonia gas also could be just released or burned to generate Nitrogen oxides; both are pulmonary chemical weapons.