gun for hire vs hit man

gun for hire and hit man both are nouns.

Word	Noun	Adjective	Verb	Adverb
gun for hire	Yes	No	No	No
hit man	Yes	No	No	No

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

Other synonyms of gun for hire include gun, gunman, gunslinger, hired gun, hitman, shooter, torpedo, triggerman.

gun for hire (noun)	hit man (noun)
a professional killer who uses a gun	a professional killer who uses a gun