good afternoon vs afternoon

good afternoon and afternoon both are nouns.

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

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

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

good afternoon (noun)	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