I've seen three GB 36' with the plates secured, two where the headliners were removed. There may be other GB's out there with a different set up, but in any case it seems to make sense that the hidden part would be secured to avoid having fasteners fall. Plus, an unsecured nut could easily just spin making it impossible to remove the bolt.
I used a similar method as described by Charlie O. Loosen all the bolts and put some no impact downward pressure on the heads. If they don't move down, the plate underneath is secure. If they do, then put a couple studs in, lift the hardware enough to get some vise grips on the stud under the hardware, remove the bolts and lift away.
J. R. Good