The Collaborative International Dictionary of English v.0.48:

GOP \GOP\ n. [from Grand Old Party.]
   The Republican Party, the younger of the two major political
   parties in the U. S.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

GOP
    n 1: the younger of two major political parties in the United
         States; GOP is an acronym for grand old party [syn:
         Republican Party, GOP]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "GOP":
   American Party, Anti-Monopoly Party, Bull Moose Party,
   Communist Party, Conservative Party, Constitutional Union Party,
   Democratic Party, Democratic-Republican Party, Farmer-Labor Party,
   Federalist Party, Free Soil Party, Greenback Party,
   Know-Nothing Party, Labour Party, Liberal Party,
   Liberal Republican Party, Liberty Party, National Republican Party,
   Populist Party, Progressive Party, Prohibition Party,
   Republican Party, Socialist Labor Party, Socialist Party,
   Socialist Workers Party, Tammany Hall, Tory Party, Whig Party,
   camp, city hall, faction, machine, major party, minor party,
   multiple party system, one-party system, opposition party, party,
   party in power, political machine, political party, splinter party,
   third party, two-party system



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

GOP
       Group Of Pictures (video, VOBU, DVD)