From what I remember of some of the vague GD-ROM tech crap that was floating around, official GD-ROMs have something like 4 or 5 sessions, with the layout looking something along the lines of:
1) Low-density session (readable by a standard CD-ROM reader, has been used e.g. for bonus desktop backgrounds)
2) Can't remember...
3) Dummy session (takes up unused space to optimize for CAV access)
4) High-density data session
5) CDDA tracks?
My memory on this is pretty vague, but some of the stuff is probably still floating around out there somewhere...