Hello all.
I've got a bit of experience with the Elder Scrolls Editor, but I'm having a bit of difficulty with the GECK. For some reason, when I modified the interior of an area, and attached my own modified sections to the existing space, they don't appear to be there.
They are, however. You just can't see them until you take the leap of faith and step through the door. Strange, yes?

Has anyone else encountered this problem before?

Yes. This is a room marker issue. If you go to the Geck tool bar under View --> Show/Hide then check 'Rooms and Portals' you will then see the big blueish boxes that are the room markers. You would have to add new room markers and portals to be able to see your additions. See the http://geck.gamesas.com/index.php/gamesas_Tutorial_Optimization for a description of Room Markers and Portals.

Yes, that's sorted it right out... thank you very much, my friend.