High ISO
In short both cameras have great high ISO performance and I'm happy with the performance of either. The GFX however exposes less light than the X1D at the same ISO. The difference seems to be 1/3 stop or less. I am hesitant to post an image because the tone curve the raw converter applies has a role here and we'll know for sure if/when DXO does their measurements.
The X1D image is clearly brighter, again not sure if this is because of the tone curve Lightroom applies or if the GFX file truly was under exposed in spite of being the same aperture, shutter speed and ISO. The noise levels look similar to me.