I realize that this is an old thread, but I too had found my GFX100 too heavy to carry on a daily basis, and when the recent GFX50r discount went live I'd ordered one as a daily or almost daily carry.
That camera, with the GF50mm onboard is very comfortable to hang from my shoulder, but the lack of stabilization had been at times problematic, as high shutter speeds and or monopod/tripod had not always been convenient. And then came the GF 45-100 F/4 OIS. The zoom has breathed new life into my relatively new GFX50r ... its OIS is nothing short of spectacular, and I find myself doing handheld imaging at 1/30sec and producing sharp images. True, it's about as heavy and long as the GF 120mm, but it sits well on the GFX50r, and of course does even better on the GFX100. But I had purchased the zoom specifically for the 50r, and that's where it will remain attached. I won't bother describing its IQ, as there are plenty of reviews on the Internet, but suffice it to say that it's exceptional.