gun down vs shoot

gun down is not a noun while shoot is a noun.

Word	Noun	Adjective	Verb	Adverb
gun down	No	No	Yes	No
shoot	Yes	No	Yes	No

As verbs, shoot is a hypernym of gun down; that is, shoot is a word with a broader meaning than gun down:

Other hypernyms of gun down include hit, pip.

gun down (noun)	shoot (noun)
	the act of shooting at targets
	a new branch

gun down (verb)	shoot (verb)
strike down or shoot down	give an injection to
	produce buds, branches, or germinate
	measure the altitude of by using a sextant
	record on photographic film
	make a film or photograph of something
	utter fast and forcefully
	fire a shot; release
	hit with a missile from a weapon
	score
	spend frivolously and unwisely
	force or drive (a fluid or gas) into by piercing
	throw or propel in a specific direction or towards a specific objective
	throw dice, as in a crap game
	variegate by interweaving weft threads of different colors
	run or move very quickly or hastily
	move quickly and violently
	send forth suddenly, intensely, swiftly
	cause a sharp and sudden pain in
	kill by firing a missile
	emit (as light, flame, or fumes) suddenly and forcefully