WordNet records no synonym and no antonym for gold standard. 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 monetary standard under which the basic unit of currency is defined by a stated quantity of gold
a paragon of excellence
“academic education is the gold standard against which other educational activity is pejoratively judged”
Synonyms, antonyms and the broader and narrower relations for gold standard come from Princeton University’s WordNet. Full sources and licences.