GH5 can never be FF. If Panasonic ever release a FF camera, they won't name it GH5 and the lens mount cannot be M4/3 as FF is too big for that mount. I doubt if Panasonic will release FF, but if they really do (and it has to be a new mount), you can say death to M4/3 mount for sure.
Mounting them won't be just tricky. It would be impossible.