I tested both the GFX and the X1D extensively. Depending upon how you prioritize your needs/wants, I think both cameras can be terrific choices. Both systems use the same sensor and have superb lenses. For the way I would use the system, photographing landscapes, the two most important priorities were (1) the system had to be as small, light and portable as possible and (2) the camera had to have a clean, simple interface that was not cluttered with deep menu systems containing a plethora of optional settings that would be irrelevant to me. As it turned out, the choice for me was easy. The X1D and its lenses are significantly smaller and lighter than the GFX and its lenses. The User Interface and the ergonomics of the X1D were exactly what I wanted. The GFX was not enjoyable to handle and operate. Too many buttons, too many options that clutter up the menus. It was, to me, everything that I don't like about Japanese cameras.
There are, of course, other differences between the two cameras that may be very important to you. The GFX uses a focal plane shutter and there are adapters that allow you to use non-native lenses. You can use Hasselblad HC lenses with the X1D, but that's it, though I should add that it's nice to have tele lenses like the HC 210 and 300 to fill in the system for now. The use of non-native lenses was not important to me and the GFX lenses are so good that I couldn't imagine not using them if I had bought the GFX. The GFX has a tilting LCD. The implementation of manual focus in live view with the X1D is much better than with the GFX. The GFX firmware is more mature. Hasselblad has made significant advances with the X1D firmware since it was introduced early this year. They are not done. There are still some missing features. However, by all accounts, it is stable at this point. Fuji is releasing lenses at a faster clip than Hasselblad. There are 5 GFX lenses now: the 32-63 zoom, the 23, the 63, the 110 and the 120 Macro. Hasselblad has a 30, a 45 and a 90. A 120 Macro will be released by year end, and a 22, a 65 and a 35-75 zoom by early 2018.
I would strongly suggest that you demo or rent each system for an afternoon and try them/hike with them.