Historically, one of the big problems with gas weapons of this kind, famously in recent history in a Russian auditorium in which large numbers of people were kept as hostages by Chechen rebels gas was used for the kind of purpose contemplated and many people died, is that there is considerable variation in individual response to a given dose of the gas, and there is also considerable variation in the amount of gas to which particular target individuals in a group are exposed due to location and countermeasures taken (e.g. breathing through a wet cloth or a gas mask). One must also face trade offs between a fast acting agent and one that is too strong to linger in the vicinity of people for too long without killing them.
An amount necessary to stop everyone is almost always enough to kill some foreseeable targets of such a gas, when a crowd may include everyone from hardy, large young men to frail elderly people and mothers with babies to people with asthma and various allergies. Any sedative that has general body effect is going to be too strong for some, and too weak for others, and indefinite suppression of bodily function with a sedative is pretty much always an unhealthy thing. Also any drug that isn't removed from a person's system by normal metabolic processes is probably a poison.
One can imagine a gas that is dosed to affect a small number of healthy young men in a fairly confined space for an hour or two that might be used, for example, in a particular wing of a prison during a riot, where perfection isn't absolutely necessary, any hostages are likely to be healthy adults, and the alternatives would be worse.
Something like the tranquilizer darts used in Mission Impossible: Rogue Nation, might be more attractive, because it would allow for deliver that is more precisely dosed (and hence easier to make fast acting without impairing non-lethality), doesn't affect innocent third parties who might be more vulnerable, is less amenable to countermeasures, and is easier to teach operators already familiar with using firearms to use.
Suspending disbelief to imagine that this would be possible, it would need to target some part of the body where body size and health doesn't impact effect, and where respiration wouldn't be impaired. For example, you'd want a highly locus specific chemical or parasite that would attach to a specific brain organ receptor and could cross the brain-blood barrier, and then degrade after perhaps a few hours or less (or after administration of an antidote if that came sooner) that is effective in very low doses.
