head gate has 5 distinct synonyms spread across 1 of its 2 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.
regulator consisting of a valve or gate that controls the rate of water flow through a sluice
a gate upstream from a lock or canal that is used to control the flow of water at the upper end
Synonyms, antonyms and the broader and narrower relations for head gate come from Princeton University’s WordNet. Full sources and licences.