It would be great in a hostage situation if such a gas existed. I don’t believe there is one that works like that.
The Russians used fentanyl. Worked like a champ. But they blew it with the people they rescued by having a terribly substandard emergency medical response. People dragged out were just laid out and simply vomited and aspirated. Great idea but weak on the critical following details.
Better to just create a diversion of some sort psychologically and kill him before he knows what happened. But the FBI will want to talk to him ABC take him alive so they can supposedly talk to him. In truth they are terrified to shoot a moslem.
The russians used some sort of gas at that theater. IIRC the gas probably killed a bunch of the hostages as well as knocking out the terrorists. Of course it was hushed up that the gas killed innocents.