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.

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