gainer has 2 distinct synonyms spread across 2 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 person who gains (gains an advantage or gains profits)
“she was clearly the gainer in that exchange”
a dive in which the diver throws the feet forward to complete a full backward somersault and enters the water feet first and facing away from the diving board
Synonyms, antonyms and the broader and narrower relations for gainer come from Princeton University’s WordNet. Full sources and licences.