Gymnasium is a Latin word (with origins in Greek), and its plural in Latin is gymnasia. But gymnasium is also an English word—one that has been in the language for centuries—and English speakers are allowed to treat it as one. In any case, the issue of gymnasium‘s plural was settled long ago; gymnasiums outnumbers gymnasia by a ratio of about 100 to one in 21st-century English-language texts that are searchable online. The ratio is roughly consistent throughout the English-speaking world.