So how long do you think the gas is going to hold out?
Judging by the recent small shtf events even approaching a gas station in a full scale SHTF will be a death sentence.
I don't have as much chlorine as I'd like, but there are ways of making it in usable quantities if you have a source of aluminum or aluminum processing dross, and I do. Likewise, the nitrogen mustard and sulfur mustard.
On the other hand, I can put a pretty sizable quantity of nitrochloroform/chlorpicrin together, should a matter of wide area pest control come up. I've not bothered with Sarin or Novichok 60/A232. Yet.