Sorry, Nikas, but if you are trying to make sense of the names that Plato gave to people and places in his tales of Atlantis, I for one cannot help, as I belong to the club of people who believe that Atlantis is nothing but a myth that Plato concocted from various other tales in order to to illustrate his political theories. On that basis, I cannot try to make educated guesses as to the origins of names such as Eumelus. In Critias, as you know, Gadeirus is the name given to Atlas's twin brother and, according to the text, his name in Greek was Eumelus, but who knows whether the latter is a translation. On the other hand, if the region of Gades has anything to do with Cadiz, the Greek name for it (i.e. other than Γάδειρα) has disappeared in the bottom of the sea, along with the legendary Atlantis.