As the title suggest a couple of things I noticed using spells that are likely otherwise not usually used:
General Gas Spells: The DC ticks down over time, which is fine but I would say depending on what is more desireable either the DC remains at caster DC until expired or choking powder does the same and ticks down, currently choking powder maintains the same DC.
Cloudkill: The duration is really short - would suggest like wall of fire etc the duration be doubled for GSF conjuration.
IMO lower DC on choking powder, its a simple free item.
The gas is pretty lethal for alot of none high fortitude.