Just what was the Gulug Stone needed for? Was it to open the door Mt. Gulug?

If that's the reason, is it elaborated on why it was stored in Oeilvert?

Thank you for your time.

Yep, that's exactly what it was for. I'm not sure if it's in the item description (I don't think there's much time to actually go in and look at it in your Key Items inventory -- could check), but anyway, in the Ultimania it's described as "An item needed to enter Gulg Volcano."

I don't think it was really elaborated on why it was stored in Oeilvert, except that maybe because of Oeilvert's magic barrier, it was to help keep people like Kuja out...? Or it was there because the ancient Terrans just kept it there for safekeeping like everything else in Oeilvert.

When you control Zidane in the Desert Palace, in the short time it takes you to reach Kuja's chambers, you can view the Gulug Stone in the Key Items section.

There a plot FAQ in Gamefaqs that might help explain why it was stored in Oeilvert. I remember reading about it years ago but I forget.

was there anything at Mt Gulug other than Hilda and an extinct race of moles???

Freeking awesome music...?

Well, okay, maybe that's just me. I love Mt. Gulug's music.

If they were extinct, then they weren't really there, were they, 1338?

let me rephrase it then ....
it was once inhabited by a species/race of moles that could write, build structures, and possibly talk... And over time, their numbers dwindled, hence driving to extinction....

I read the FAQ, and it gave a pretty good description.

In Madain Sari, it states that Esto Gaza had summoners (there's a lot more than that, but for the sake of length I'll leave it at that) and since Garland feared the eidolons he sealed the remnants of the summoners off and it was, like you guys said, stored their for the magic barrier (although I think the gulug stone itself was a sort of magic inhibitor) to seal protect the key to the mountain.