In Oielvert, I’m assuming that you already lit all the orbs and talked to the faces, right? From there, just go back out into the same room where you first entered. There should be a door all the way to the right. If you go into that room, you’ll see another globe and probably the moogles.
Then just keep walking to the right into the next room. From there, it’s pretty straightforward. You’ll find a transport, and it’ll take you down to the find the Gulug Stone.
Hope this helps a little 🙂