If you need long teles and are not too concerned about corners, I would get an af-adapter and a good Canon tele. Those teles are top and cheaper than good mf teles. Also the af is good with the adapter.
Another possibillity is a Contax 645 apo 350mm with an af adapter from gfx to Contax 645.
Nice and sharp with good corners but much slower af than the Canons.
If you just need mf there are many options:
Very good: Hasselblad cfi 350mm, Hasselblad 250mm cfi, Contax 200mm apo sonnar for c/y. Made for a smaller format,
but I have tested it and is works very very well on the gfx (very heavy)
Medium good: Mamiya 300mm and 200mm. Pentax mf teles.
Hasselblad 350 fe teles are not in the same class.
Also if you need longer leles, the Canon 800mm manual focus is very very good. But just on a tripod!!
Not bad corners.
From what I have tested, I would say the Contax Apo-sonnar 200mm f.2 is better than everything else.
Just very heavy and mf. Canon 200mm f.1.8 should be very close and does have af. If used wide open that is a BIG +. I have not used this lens, but I have heard very good things about it.