User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7742465-20130605220952/@comment-3146930-20130606005220

Her name can be pronounced a number of ways, as can a lot of other characters (Thalia is apparently pronounced about 20 different ways). I call her Guy-ah as that is how they say it in Age of Mythology. Gee-ah is just another way to say it and isn't the completely official way to say it, as Rick Riordan has admitted to not really listening to the audiobook.