WordNet records no synonym and no antonym for roundup. 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.
the activity of gathering livestock together so that they can be counted or branded or sold
a summary list; as in e.g. "a news roundup"
the systematic gathering up of suspects by the police
“a mass roundup of suspects”
Synonyms, antonyms and the broader and narrower relations for roundup come from Princeton University’s WordNet. Full sources and licences.