The gas gun is the Green Hornet's weapon. It shoots out a harmless gas that knocks his enemies unconscious so that they can later be found with incriminating evidence and easily collected by the police. It also serves the purpose of scaring crooks who mistake it for a lethal firearm.
In the 2011 Green Hornet movie, Kato makes the gas gun for Britt because Britt had no previous fighting experience. It fires gas cartridges that had the ability to knock someone unconscious for eleven days until Kato used it on Britt, at which point Kato changed the effect to last for one hour.