guarantee has 10 distinct synonyms spread across 6 of its 7 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.
promise to do or accomplish
“guarantee to free the prisoners”
an unconditional commitment that something will happen or that something is true
“there is no guarantee that they are not lying”
a collateral agreement to answer for the debt of another in case that person defaults
Synonyms, antonyms and the broader and narrower relations for guarantee come from Princeton University’s WordNet. Full sources and licences.