WordNet records no synonym and no antonym for gaff. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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 sharp metal spike or spur that is fastened to the leg of a gamecock
a spar rising aft from a mast to support the head of a quadrilateral fore-and-aft sail
an iron hook with a handle; used for landing large fish
Synonyms, antonyms and the broader and narrower relations for gaff come from Princeton University’s WordNet. Full sources and licences.