WordNet records no synonym and no antonym for game plan. It is the only member of each of its synsets, 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.
(figurative) a carefully thought out strategy for achieving an objective in war or politics or business or personal affairs
“newscasters speculated about the President's game plan for an invasion”
(sports) a plan for achieving an objective in some sport
Synonyms, antonyms and the broader and narrower relations for game plan come from Princeton University’s WordNet. Full sources and licences.