I doubt there’s a 35mm fisheye with the image circle needed to cover the GFX sensor. I wish someone would make an enlarging lens adapter, like a reverse SpeedBooster, even if it would rob a stop of light. UPDATE: Laowa makes one: https://www.venuslens.net/product/laowa-magic-format-converter-mfc/ but with no electronic connection to the camera, the EF version I’d need has no aperture control (The Nikon version has it, though)
Laowa also recently introduced GFX dedicated 17mm f4 (~13mm on GFX) - not a fisheye, no AF but 115 degrees angle of view. (Not shilling for Laowa - never even tried any of their lenses)
Here’s another idea: a SpeedBooster for GFX so you could use older *real medium format lenses without cropping. Mmmm... the Hasselblad 30mm f3,5 on a GFX… https://www.flickr.com/photos/dorkstar/5933520512/in/photolist-a3gYzT-a6X3k5-a3jQVN (A skate photo I made with a 503CW and the H-blad fisheye)