I still have my 500/4G from when I was a Nikon shooter, and IIRC when I've tried using it with the GFX 100 manually (not super practical, unsurprisingly) it covered enough of the sensor that I'd have more megapixels using it with the Fringer than buying/renting a [45mp] Z8 to use natively. Of course I know it's not gonna be "football and BIF" performance, but is it gonna be "grounded birds" performance or "inanimate objects" performance?
Can't answer your question directly, Lee, but can offer this. I got the Fringer adapter so I could use my Nikon 500pf with my Fuji X-T3. Initially wasn't too pleased with the performance, but after a couple of firmware updates from Fringer the combo performed very well.
Yes there was some AF lag when trying to shoot birds in flight, but honestly some of that was attributable to the Fuji's AF which was never as good as my Nikon AF to begin with. That said, the combo did work very well for coyotes and such that I encountered in the field. Also was pleased with Fringer for coming out with their updates as quickly as they did. Made my shooting experience so much better.