WordNet records no synonym and no antonym for goalpost. 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.
one of a pair of posts (usually joined by a crossbar) that are set up as a goal at each end of a playing field
Synonyms, antonyms and the broader and narrower relations for goalpost come from Princeton University’s WordNet. Full sources and licences.