Howdy!
There are actually three different mechanisms I used for knockout gas in my own *Deeds Not Words.*
1. Traditional: "Fort Save DC <x>, Initial/Secondary Damage unconsciousness," although this is the rarest sort.
2. Depressant chemicals reduce Dex temporarily-- upon reaching 0 Dex, a character passes out. Consider it a non-lethal Dex poison ala fantasy d20. This is the most common sort of incapacitory agent encountered in DNW. I like this approach because a character can suffer some impairment from a partial dose without passing completely out, which is how anesthetics function in the real world.
3. The anti-superbeing knockout gases used by the Special Executive for Parahuman Affairs (SEPA) in the *Veritas, Vigilantia, Necessitas* supplement deal subdual damage on a per-round basis if someone in the cloud fails a Fort Save. They're nicknamed "horse gas" and "elephant gas" for their relative potency.
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An ordinary human (and most 1st level characters) exposed to such stuff is going to last about one point five seconds.
I should add that these two potent chemicals cause unfortunate side effects in beings with less-than-superhuman Con scores, due to respiratory arrest and system shock when large, uncontrolled doses are taken in. The Russian theater assault with Fentanyl gas a few months back is a perfect illustration of why the "magic knock-out gas" ubiquitous in the comics is rarely used by law enforcement agencies in real life.
*Veritas, Vigilantia, Necessitas* deals with the trouble of using powerful sedative gases in tactical situations and offers a variety of equipment for dispersing it, though it's all intended for use in a contemporary-era campaign.
Cheers!
SL