gear box vs gearbox

gear box and gearbox both are nouns.

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

As nouns, gear box and gearbox are synonyms defined as:

gear box and gearbox: the shell (metal casing) in which a train of gears is sealed

Other synonyms of gear box include gear case.

gear box (noun)	gearbox (noun)
the shell (metal casing) in which a train of gears is sealed	the shell (metal casing) in which a train of gears is sealed