WordNet records no synonym and no antonym for gettysburg address. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a three-minute address by Abraham Lincoln during the American Civil War at the dedication of a national cemetery on the site of the Battle of Gettysburg (November 19, 1863)
Synonyms, antonyms and the broader and narrower relations for gettysburg address come from Princeton University’s WordNet. Full sources and licences.