ground floor has 2 distinct synonyms spread across 1 of its 2 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.
the floor of a building that is at or nearest to the level of the ground around the building
becoming part of a venture at the beginning (regarded as position of advantage)
“he got in on the ground floor”
Synonyms, antonyms and the broader and narrower relations for ground floor come from Princeton University’s WordNet. Full sources and licences.