mass meeting vs gathering

mass meeting and gathering both are nouns.

Word	Noun	Adjective	Verb	Adverb
mass meeting	Yes	No	No	No
gathering	Yes	No	No	No

As nouns, gathering is a hypernym of mass meeting; that is, gathering is a word with a broader meaning than mass meeting:

Other hypernyms of mass meeting include assemblage, assembly.

mass meeting (noun)	gathering (noun)
a large gathering of people intended to arouse enthusiasm	the social act of assembling
	the act of gathering something
	sewing consisting of small folds or puckers made by pulling tight a thread in a line of stitching
	a group of persons together in one place