hitman vs gun for hire

hitman and gun for hire both are nouns.

Word	Noun	Adjective	Verb	Adverb
hitman	Yes	No	No	No
gun for hire	Yes	No	No	No

As nouns, hitman and gun for hire are synonyms defined as:

Other synonyms of hitman include gun, gunman, gunslinger, hired gun, hit man, shooter, torpedo, triggerman.

hitman (noun)	gun for hire (noun)
a professional killer who uses a gun	a professional killer who uses a gun