WordNet records no synonym and no antonym for half gainer. 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 dive in which the diver throws the feet forward and up to complete a half backward somersault and enters the water facing the diving board
Synonyms, antonyms and the broader and narrower relations for half gainer come from Princeton University’s WordNet. Full sources and licences.