sleepover vs social gathering

sleepover and social gathering both are nouns.

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

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

sleepover: an occasion of spending a night away from home or having a guest spend the night in your home (especially as a party for children)
social gathering: a formal or official social gathering or ceremony of people

Other hypernyms of sleepover include affair, function, occasion, social affair, social function, social occasion.

sleepover (noun)	social gathering (noun)
an occasion of spending a night away from home or having a guest spend the night in your home (especially as a party for children)	a formal or official social gathering or ceremony of people