go down vs decline

go down is not a noun while decline is a noun.

Word	Noun	Adjective	Verb	Adverb
go down	No	No	Yes	No
decline	Yes	No	Yes	No

As verbs, go down and decline are synonyms defined as:

Other synonyms of go down include wane.

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

go down (noun)	decline (noun)
	a downward slope or bend
	a gradual decrease; as of stored charge or current
	change toward something smaller or lower
	a condition inferior to an earlier condition; a gradual falling off from a better state; decline

go down (verb)	decline (verb)
grow smaller	grow smaller
stop operating	grow worse
be defeated	show unwillingness towards
be ingested	inflect for number, gender, case, etc.
go under	go down
disappear beyond the horizon	fall in value
move downward and lower, but not necessarily all the way	not accept as true
be recorded or remembered