No, no, and no. Chemical weapons are impossible to stop, even now. The best you can really hope for is that the person using them is retarded - and they normally are. Some aspects of MAD apply, and they are rather difficult to actually produce, store and distribute - but there's no immediate "line of defence" against modern chemical weaponry outside of those which can target the distribution - anti-rocket defences, for example.
The masks may be handy, but to my knowledge, nerve gases are a chemical which disrupts nerves themselves. They don't necessarily need to be inhaled, only dissolved into fluids on the surface of the skin. Once it does, it can travel very quickly.
This is why I mentioned the eyes, actually - it's a common misconception that gas masks are for the respiratory factor, but they are just as important in protecting the eyes. When dissolved into the fluids of the eyes, the nerve agent will paralyze the eye muscles, and the muscles surrounding the eyeballs - meaning that although you can still see, you struggle to move your eyes at all in any independent fashion from the rest of your head.
The nightmare scenario, here, is that Ukraine will employ "duster drones" to sweep across Russian positions in a precise, concentrated fashion - citing "legitimate defensive use" and essentially, daring Russia to escalate to a full nuclear assault. All while the West backs it up, saying that WMDs are only illegitimate in a handful of contexts (like tear gas) and that this is a legitimate tactical use.
