afternoon vs good afternoon

afternoon and good afternoon both are nouns.

Word	Noun	Adjective	Verb	Adverb
afternoon	Yes	No	No	No
good afternoon	Yes	No	No	No

As nouns, afternoon and good afternoon are synonyms defined as:

afternoon and good afternoon: a conventional expression of greeting or farewell, used to wish someone a good afternoon

afternoon (noun)	good afternoon (noun)
a conventional expression of greeting or farewell, used to wish someone a good afternoon	a conventional expression of greeting or farewell, used to wish someone a good afternoon
the part of the day between noon and evening