casing vs gear box

casing and gear box both are nouns.

Word	Noun	Adjective	Verb	Adverb
casing	Yes	No	No	No
gear box	Yes	No	No	No

As nouns, gear box is a hyponym of casing; that is, gear box is a word with a more specific, narrower meaning than casing:

Other hyponyms of casing include boot, gear case, gearbox, jacket.

casing (noun)	gear box (noun)
the enclosing frame around a door or window opening	the shell (metal casing) in which a train of gears is sealed
the housing or outer covering of something
the outermost covering of a pneumatic tire