good afternoon vs farewell

good afternoon and farewell both are nouns.

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

As nouns, farewell is a hypernym of good afternoon; that is, farewell 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
farewell: an acknowledgment or expression of goodwill at parting

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

good afternoon (noun)	farewell (noun)
a conventional expression of greeting or farewell, used to wish someone a good afternoon	the act of departing politely
	an acknowledgment or expression of goodwill at parting