It is ...... on the GS1. On the RZ67 i find the centerline with a pencil on the table, place the camera on the pencil, where the camera balances, I have the centerline of gravity, then I do the same thing once more, just after I have rotated the camera 90 degrees. Now those pesky small japaneese have been so cleaver that they put the screw mounting hole where those 2 centerlines converge, ain't they clever
Just to try your theory, I dismounted the mounting part on top of the ball, put the ball vertically, removed all tension on the ball, and the ball is still standing straight up after 2 minuttes, in the center of the tripods legs, so unless gravity is very warped where I sit, the center of gravity of camera, tripod and ballhead is in line, and on top of each other.