The only free and comprehensive online etymological dictionary of the Spanish language
gimnasio m. (Noun) "gymnasium"
17th cent. Borrowed form Latin gymnasium 'id.,' itself borrowed from Ancient Greek γυμνάσιον ‎(gumnásion) 'id.' Formed from γυμνός (gumnós) "naked," as exercises were performed nude. From Proto-Indo-European *nogw-no- 'id.' (whence nudo (1)).