"Luna and I can stand at either end of the corridor ...and warn people not to go down there because someone's let off a load of Garrotting Gas. ...Fred and George were planning to do it before they left."
-- Ginny Weasley (OP32)
Colorless gas which apparently knocks people unconscious.
References from the canon
Fred and George were planning to release Garrotting Gas in a corridor, but left school before they had a chance to carry out the plan.Ginny seized upon the idea as a diversion to keep Umbridge away from her office. (OP32)
Kill (someone) by strangulation, especially with a length of wire or cord (Oxford Dictionary)
It seems highly unlikely that Fred and George would have released something which was actually deadly, so we assume that Garrotting Gas is non-lethal