User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-6064271-20130807130754/@comment-3146930-20130807215220

Ephialtes and Otis during the first war were not actually children of Gaea, but the children of Iphimedeia and Poseidon (or Iphimedeia and Aloeus). Anyway, the aren't really true Gigantes and only tried to help Gaea by piling mountains to reach Olympus. This would mean they didn't have a bane to fight in the first war.

However, their back-story is kind of different in the series. It is possible that after the war, they became jealous of Dionysus and wanted to take his place as the Party god. Thus, making them banes of the new god Dionysus.