So here is a mock-up of the 500PF and Gripped D850... Apologies for the cell phone pics. Cheaper gimbal but effects of gravity and balance should be pretty much the same
. Had to essentially shove the foot all the way forward in the cradle (essentially right up against the safety stop pin) to balance and have any semblance of the rig staying in position when tilted, but it works. Right hand comes pretty close to adjustment knob, but it's manageable - although other gimbals may differ depending on knob/cradle config. I had planned to switch to the RRS cradle when I decided on which gimbal I wanted to buy (looking at Siriu, Benro, Jobu, Wembely, and PMG) so will have to pay attention to that since the RRS knob is sizeable.
Taking of the grip helps, but of course then FPS drops. I suppose I could remove the L-Bracket when I know I'm shooting with the gimbal, as it's a pretty substantial weight surprisingly which would help - either option (or both) would allow one to bring the lens back in the cradle. Leaning toward the latter will do some more testing.
Edit - adding flash would likely help as well, as typically any flash and bracket weight would be forward of the balance point, at least if using one that mounts via the lens foot ie the RRS, Wimberly or Custom Brackets bottom mount setups.
Edit 2 - tried without L bracket, surprisingly there was little change, although I was able to slide the plate back about 3/8" or so. Taking off the Grip altogether does allow a more "normal" mounting of lens plate being centered for lack of a better description within the plate.
It just appears with the the lighter PF there is no strait forward way to balance as one would with a heavier long telephoto prime. That being said, it's entirely workable/usable, just a different setup than one might be used to with heavier/longer glass. I have a Wimberly Flash bracket coming so will see how much a flash affects the balance as well.