grotesque has 4 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.
distorted and unnatural in shape or size; abnormal and hideous
“tales of grotesque serpents eight fathoms long that churned the seas”
art characterized by an incongruous mixture of parts of humans and animals interwoven with plants
Synonyms, antonyms and the broader and narrower relations for grotesque come from Princeton University’s WordNet. Full sources and licences.