Hydrogen, e.g. (7)
(Other definitions for gas that I've seen before include "Petrol (USA)" , "See 2" , "Eg, oxygen" , "Domestic fuel" , "Chatter vapidly" .)
(Other definitions for element that I've seen before include "of the weather" , "He or I maybe" , "Constituent -- habitat" , "Chemical building-block" , "Constituent part (7)" .)