Were you using The Mammoth Book of British Kings and Queens?
In it, Eleuther, or Eliffer, is the son of Gurgust, father of Peredur. (There's a different Elidyr who is the son of Morvidus.)
In Geni, he is Eliffer Gosgorddfawr "of The Great Army" ap Athrwys, King of Ebrauc, not the son of Gwrast ap Ceneu.
What I can find so far on Gwrast gives him only the two sons; he divided his kingdom, north and south, between them.