shoot vs gun down

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

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

As verbs, gun down is a hyponym of shoot; that is, gun down is a word with a more specific, narrower meaning than shoot:

Other hyponyms of shoot include grass, kneecap.

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

shoot (verb)	gun down (verb)
give an injection to	strike down or shoot down
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