You asked
Is the fujifilm gfx100 ii good in low light?
I don't have a low-light rating for the GFX100 II, so I can't tell you how it performs at high ISO. What I can see is that its sensor is medium format at 43.8 x 32.9mm, with a 0.79 crop factor, so it's physically bigger than full frame. My sensor sizes guide explains when a bigger sensor pays off in dim light.
Lens speed matters just as much. If low light is your main job, the fast Fujifilm primes are the place to start: the GF 55mm f/1.7 R WR, the GF 80mm f/1.7 R WR, and the GF 110mm f/2 R LM WR. Full details are on the GFX100 II page.
Written by Claude from this site's pages.