I have been using a lot of adapted glass on my GFX lately and it is surprising how many lenses cover the GFX well with minimum or no vignetting. Fringer just released a video with the Tamron SP 150-600mm F/5-6.3 Di VC USD G2 showing off its AF performance and explaining how to make the 150-600mm cover the GFX without vignetting. According to Fringer you have to remove a plastic ring from the back of the lens to remove the vignetting at the wide end of the zoom range and it shouldn’t require any tools to complete.
The video came out a few days after Fringer updated us about its work on a GFX AF adapter, but it is making me wonder if maybe Tamron might repurpose some full-frame lenses for GFX with built-in AF adapters like Sigma did with Sony since Tamron just started making lenses for Fujifilm cameras.