Republican Party vs GOP

Republican Party and GOP both are nouns.

Word	Noun	Adjective	Verb	Adverb
Republican Party	Yes	No	No	No
GOP	Yes	No	No	No

As nouns, Republican Party and GOP are synonyms defined as:

Republican Party and GOP: the younger of two major political parties in the United States; GOP is an acronym for grand old party

Republican Party (noun)	GOP (noun)
the younger of two major political parties in the United States; GOP is an acronym for grand old party	the younger of two major political parties in the United States; GOP is an acronym for grand old party