As a user of Contax C/Y glass, I'm aware of an official adapter for G bodies. Naturally, you must focus by scale and - at least for unusual focal lengths - use an external VF. For reasons of selection and affordability, I rank C/Y pretty low. You can, however, use other brands like Tamron by stacking another adapter.
The entire enterprise strikes me as rather hokey. Better to buy a native camera body for the optic of interest.