User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7594114-20130621070430/@comment-3146930-20130622114524

They mean in the books. Gaea can be spelled Gaea, Gaya, Ge, Gea, or Gaia, but in the American versions of the books, it is Gaea. In the UK books, it is Gaia. Gaia is considered the more popular spelling overall however.