gun has 18 distinct synonyms spread across 5 of its 8 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 weapon that discharges a missile at high velocity (especially from a metal tube or barrel)
a professional killer who uses a gun
a hand-operated pump that resembles a pistol; forces grease into parts of a machine
the discharge of a firearm as signal or as a salute in military ceremonies
“two runners started before the gun”
Synonyms, antonyms and the broader and narrower relations for gun come from Princeton University’s WordNet. Full sources and licences.