Yes, the Vello Two-Axis Hot-Shoe Bubble Level will fit the hot shoe of the GFX system cameras.

this one fits super nice on my gh-5, will grab some more as soon as they go on sale!

i thought Hot shoe were standard sizes and i assume that it will otherwise 3rd party flash manufacturers would not be able to sell flashes

I can't imagine why it wouldn't. It just needs to slide into the hotshoe. It fits my Sony cameras no problem. Sometimes I also just drop it onto the LCD when doing and overhead shot. Generally, just super handy to have a small level you can use on the hotshoe or on the camera itself.

I, too, have no idea what the liquid is. I did like the responder citing spirit levels.. I bought it because my camera base covered the level on my tripod head. I like the way it works on the hot shoe....

There are no metal or electrical connections, the vials are full or fluid that doesn't leak out, and it is otherwise nothing but plastic, so Yes. However, the hot-shoe itself is an electrical contact, and so, isn't waterproof. That would be the point of concern for the safety of the camera.

I think it would fit. I bought one for my Panasonic micro four thirds and found out that the camera shows you the horizon line. In fact I havent used it yet and dont need it for the pictures I take. Maybe it is very good for a special kind of photography, but I dont know which one It is. I dont think it is a practicall accesorie