I think that when both sides use poison gas in warfare, the net effect is that everyone's soldiers end up having to fight while wearing rubber suits, which offer an effective defense against gas but are damn inconvenient to fight in. So it just ends up making things worse for everyone. Furthermore, being the first to use poison gas, before your enemy starts to defend against it and retaliate in kind, doesn't really provide that big of an advantage. In the end, I guess that the reason gas was successfully banned after WWI was that everyone involved agreed that it was more trouble than it was worth.

I suppose that, even in warfare, not everything is zero-sum.