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]