WordNet records no synonym and no antonym for gateway drug. It is the only member of its synset, 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 habit-forming drug that is not addictive but its use may lead to the use of other addictive drugs
“one college athlete recently called beer a gateway drug for young people”
Synonyms, antonyms and the broader and narrower relations for gateway drug come from Princeton University’s WordNet. Full sources and licences.