past vs good old days

past and good old days both are nouns.

Word	Noun	Adjective	Verb	Adverb
past	Yes	Yes	No	Yes
good old days	Yes	No	No	No

As nouns, good old days is a hyponym of past; that is, good old days is a word with a more specific, narrower meaning than past:

past (noun)	good old days (noun)
a verb tense that expresses actions or states in the past	past times remembered with nostalgia
the time that has elapsed
a earlier period in someone's life (especially one that they have reason to keep secret)

past (adjective)	good old days (adjective)
earlier than the present time; no longer current
of a person who has held and relinquished a position or office

past (adverb)	good old days (adverb)
so as to pass a given point