stomach flu vs gastroenteritis

stomach flu and gastroenteritis both are nouns.

Word	Noun	Adjective	Verb	Adverb
stomach flu	Yes	No	No	No
gastroenteritis	Yes	No	No	No

As nouns, stomach flu and gastroenteritis are synonyms defined as:

stomach flu and gastroenteritis: inflammation of the stomach and intestines; can be caused by Salmonella enteritidis

Other synonyms of stomach flu include intestinal flu.

stomach flu (noun)	gastroenteritis (noun)
inflammation of the stomach and intestines; can be caused by Salmonella enteritidis	inflammation of the stomach and intestines; can be caused by Salmonella enteritidis