Yay, I can finally answer this. ^^ There's a Skit in the storyline segment that I'm working on right now where Shaorune indirectly addresses this. Basically, he explains that the spirit/soul (haven't decided on how to translate this particular word yet) of a Sacred Beast is immortal so if that is revived, then the body can be regenerated. So Geyorkias presumably made himself another body after being revived at Karegia Castle. I'm still a tad confused on how his first revival was incomplete at this point though, but maybe the game will get into that later.
So that is how it works...
Thanks lanyn!