auld langsyne vs good old days

auld langsyne and good old days both are nouns.

Word	Noun	Adjective	Verb	Adverb
auld langsyne	Yes	No	No	No
good old days	Yes	No	No	No

As nouns, auld langsyne and good old days are synonyms defined as:

Other synonyms of auld langsyne include lang syne, langsyne, old times.

auld langsyne (noun)	good old days (noun)
past times remembered with nostalgia	past times remembered with nostalgia