On the GFX 50s, you can double tap the LCD, like on a P1 back, this brings you to 100% view (on the 50s I always felt it was an overzoom past 100%),
You can expand/shrink the LCD like on the iPhone (assume the 100s allows this)
If on the EVF you could push one of the dials, I believe the rear one to zoom to 100%, I don't use the EVF at 100% very often due to same issue overzooming.
Paul C