gear has 11 distinct synonyms spread across 5 of its 5 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 toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion
wheelwork consisting of a connected set of rotating gears by which force is transmitted or motion or torque is changed
“the fool got his tie caught in the geartrain”
a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)
equipment consisting of miscellaneous articles needed for a particular operation or sport etc.
set the level or character of
“She pitched her speech to the teenagers in the audience”
Synonyms, antonyms and the broader and narrower relations for gear come from Princeton University’s WordNet. Full sources and licences.