GP vs Dr.

GP and Dr. both are nouns.

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

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

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

GP (noun)	Dr. (noun)
a physician who is not a specialist but treats all illnesses	a licensed medical practitioner
	a person who holds Ph.D. degree (or the equivalent) from an academic institution