Most of the old cameras I have worked on have the GG inserted into the frame from the film side, in which case the glass must be positioned so that the inner (frosted) surface of the glass is the same distance from the camera back as the film will be. In 4x5s, that's 0.195" or 5 mm (if I remember correctly).

When I build my second 4x5 camera, I made my GG frame differently, so that the glass is inserted from the BACK of the camera. That way the GG frame provides the spacing and the thickness of the glass is irrelevent. Don't know why the camera makers didn't do it the same way . . .