GOP vs Republican Party

GOP and Republican Party both are nouns.

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

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

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

GOP (noun)	Republican Party (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