Thank you.I’m going to guess that the IBIS in the GFX100 will function much like my X-H1. That being the case, even when IBIS is turned off, it is still “powered up” when the camera is turned on - which positions the sensor accurately, and “holds it in place,” even though it won’t be moving to stabilize the image. Eg., you can’t clean the sensor with the camera turned off because the sensor will “move around.” So, you power up the camera but turn the IBIS off so that the sensor is “held still in place” for cleaning.
So, I suspect the answer to the orientation question is that with the camera on, it doesn’t matter which way the camera is pointed, re up, down, straight-down, etc., because the sensor is being “held in place” using it’s suspension system even if IBIS is not active. Hope that makes sense.
Rand