In WWI gas was much, much more likely to cause a soldier to leave the line for a while than it was to kill them. I think the whole 'they will have to put on masks' aspect mattered a lot more.
Phosgene killed 100k between 1915-1918. In a place where everyone was crammed into a tiny area and huge amounts (and concentrations) of the stuff was used at the same time. For each individual bombardment of a sector 10kms long, at best maybe 100-200 people would have died. (1000 bombardments actually seems like an extremely small amount).
Sarin and Tabun are worse, but not so much worse that the low level of concentration that an aerial bombing raid can provide is enough to kill thousands. I would say a fleet of Heinkels or Junkers' would probably cause a couple of dozen deaths, along with a lot of disruption. This is assuming the planes can drop enough gas to rival a Great War bombardment, which is unlikely.
- BNC