Traditionally, you can adapt lenses made for a larger format on a smaller format — full frame lenses on an APS-C sensor, for example — but going the other way will produce issues the majority of the time. But thanks to Metabones, you can now adapt any Canon lens to a Fujifilm medium format camera with full functionality and no loss of image quality.
The usual problem is that a lens designed for a smaller format system will not project a large enough image circle to cover a larger sensor. However, the new Metabones EF-GFX Smart Expander gets around this in a clever way, by magnifying the image circle from a Canon EF lens to cover the sensor of any Fujifilm GFX medium format camera. The tradeoff is a 1.26x crop factor that increases the focal length of whatever lens you are using and decreases its maximum aperture. For example, 24-70mm f/2.8 lens will become a 30-88mm f/3.5 zoom or a 50mm f/1.4 lens will become a 63mm f/1.8. The adapter also includes electronic integration to allow for autofocus and in-body stabilization. Metabones does warn that autofocus may "have unsatisfactory performance and may not work at all with some lenses," however. Still, for any Fuji GF system user who wants access to Canon's deep library of lenses and who doesn't need extreme autofocus, it is exciting news!