Here's one 645 to GFX adapter-
And another one-
KIPON Adapter for Mamiya 645 M645 Mount lens to Fuji Fujifilm GFX mount Medium Format camera
I have a Pentax 645 to GFX adapter, Kipon brand, and it works fine. I simply use the lens manually, both exposure and focus. I don't know if there are any adapters that will communicate with the lenses, either Pentax or Mamiya. As long as a full manual camera will work for you, you'll be fine. The Fuji can be set to still provide metering; you might not to change a setting in the menu to have it register manual aperture, I forget.
Post #22 in this thread talks specifically about the 80mm and an adapter. I haven't read the thread. I seem to remember that I found enough info to dive into the Pentax 645 lenses and adapters. It is mainly focused on a digital camera and we aren't allowed to talk about those things here so I'll let you do your own searching in that forum.
I just got my GFX100S yesterday. Still trying to figure out the settings/menus. I have attached a Nikon 105mm lens using a Fotodiox adaptor, and I am noticing that I am not seeing evidence of the wonderful image stabilization I had heard all about on this camera. Now - I cant even find any menu...