good afternoon vs greeting

good afternoon and greeting both are nouns.

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

As nouns, greeting is a hypernym of good afternoon; that is, greeting is a word with a broader meaning than good afternoon:

good afternoon: a conventional expression of greeting or farewell, used to wish someone a good afternoon
greeting: (usually plural) an acknowledgment or expression of good will (especially on meeting)

Other hypernyms of good afternoon include farewell, word of farewell, salutation.

good afternoon (noun)	greeting (noun)
a conventional expression of greeting or farewell, used to wish someone a good afternoon	(usually plural) an acknowledgment or expression of good will (especially on meeting)