social affair vs gathering

social affair and gathering both are nouns.

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

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

Other hypernyms of social affair include social event, assemblage.

social affair (noun)	gathering (noun)
a formal or official social gathering or ceremony of people	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