gill has 2 distinct synonyms spread across 2 of its 4 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 British imperial capacity unit (liquid or dry) equal to 5 fluid ounces or 142.066 cubic centimeters
a United States liquid unit equal to 4 fluid ounces
any of the radiating leaflike spore-producing structures on the underside of the cap of a mushroom or similar fungus
respiratory organ of aquatic animals that breathe oxygen dissolved in water
Synonyms, antonyms and the broader and narrower relations for gill come from Princeton University’s WordNet. Full sources and licences.