GH5 has exactly same GH4 size and is compatible with all existent cages
Image courtesy: Pier-Yves Menkhoff (Weddingetcetera.Com).
An additional info on the new GH5: At Salon de La Photo Panasonic confirmed the GH5 has the same GH4 size. This means all current GH4 cages will work on the new GH5 too.
You can see a couple of moew GH5 and Leica lens images at our GH5 facebook group.