affair vs gathering

affair and gathering both are nouns.

Word	Noun	Adjective	Verb	Adverb
affair	Yes	No	No	No
gathering	Yes	No	No	No

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

Other hypernyms of affair include social event, assemblage.

affair (noun)	gathering (noun)
a vaguely specified concern	the social act of assembling
a formal or official social gathering or ceremony of people	the act of gathering something
a usually secretive or illicit sexual relationship	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