gastroenterologist vs Dr.

gastroenterologist and Dr. both are nouns.

Word	Noun	Adjective	Verb	Adverb
gastroenterologist	Yes	No	No	No
Dr.	Yes	No	No	No

As nouns, Dr. is a hypernym of gastroenterologist; that is, Dr. is a word with a broader meaning than gastroenterologist:

gastroenterologist: a physician who specializes in diseases of the gastrointestinal tract
Dr.: a licensed medical practitioner

Other hypernyms of gastroenterologist include doc, doctor, MD, medico, physician.

gastroenterologist (noun)	Dr. (noun)
a physician who specializes in diseases of the gastrointestinal tract	a licensed medical practitioner
	a person who holds Ph.D. degree (or the equivalent) from an academic institution