User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5247339-20130623145220/@comment-3146930-20130623160130

As far as the Gigantes, in the original myths they were placed in various prisons or killed. I don't know if they would go to Tartarus, otherwise they would have reformed right away.

Maybe because they are children of Gaea, when they are killed they return to the earth. It makes sense as Porphyrion and Alcyoneus both rose up from the earth when they reformed. Enceladus claims that with the help of Gaea, he clawed his way out from under a mountain. Otis and Ephialtes as well never left their prison and instead stayed underground.