'Gas' is short for either 'knockout gas' or 'sleeping gas', whichever term you prefer. It is a gentle KO method, involving the victim inhaling some sort of anesthetic gas. The gas should then put the victim to sleep.
Some scenes may contain 'poison gas' or 'tear gas', which do not contain a sleepy element. Those scenes are rare.
Gas scenes are fairly popular on Sleepycomics.com.