First up, Narcojet is an injection-vector toxin and it doesn't have a rating. I think you mean Neuro-Stun IX, which has a contact/inhalation-vector.
1. The people are exposed to the toxin right away, as soon as they enter the gas cloud (or the gas cloud surrounds them) they are exposed to it, and therefore they will be affected by it.
2. You roll the Toxin Resistance test at the end of the Combat Turn if the speed is Immediate or delayed by the number of Combat Turns indicated in the Speed. Neuro-Stun has a Speed of 1 Combat Turn, therefore the targets would need to make the resistance test at the end of the next Combat Turn.
3. A gas mask or respirator would not help someone who is already exposed, but it could potentially help someone if they went running into the cloud. Putting on a Gas Mask after you've breathed in the gas is pretty much useless. Against Neuro-Stun specifically, because it is a Contact-vector as well as Inhalation, it can be argued that neither of these protections would help because you aren't protected on the contact vector. Now, Chem-protection (Armor mod) plus Respirators would mean you're covered on both fronts. Some GMs might let you use your higher resistance bonus, but I think it is more likely you need to resist with the lowest of the bonuses.
An antidote patch, however, could be applied before you make the resistance test to give its bonus on the resistance test. That's actually the whole point of the antidote patch. It is specifically mentioned in both the Antidote Patch and Antidotes in the Toxin section that the window to apply an Antidote is very small. It must be applied before the toxin has taken effect, and if you aren't applying it until after being exposed, that leaves only a few seconds of a window. Remember, Antidote patches also work for 20 minutes if you know what you're going up against.
Extra. I also want to point out that gas grenades aren't necessarily immediate, depending on how the grenade is set up. For example, if they are timed, rather than impact triggered, then it might take a couple seconds before the grenade goes off. Tactically speaking, for example, if a team frequently uses gas grenades they might keep them on timed triggers to give other team members the chance to put on their gas masks. That way, grenade goes out, rest of team sees it, puts on gas masks, then it goes off and the team is safe. If the team were to put on the gas masks first, it might warn whoever is about to get gassed and they might focus fire on the grenade holder, or something similar.