I use my H3D-31 on a Fuji GX680.
Adapter plates are available on ebay for the H back, but it would probably be best to try to find a Kapturegroup plate. I was lucky enough to buy one from a member on the GX680 facebook group.
You can directly connect the back to the flash sync port, but you do have to set the exposure time in the menus to match the camera shutter. However, I modified my 680 to add a 3.5mm jack connected directly to the lens x-contact, which now does not require any time setting in the back.
Other than that, it's dirt simple to use on the GX680.