good morning vs greeting

good morning and greeting both are nouns.

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

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

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

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

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