The Contax G mount has a flange distance of 29mm. As this is shorter than the flange distance of common SLR mounts like the EF and F mount, lenses for the Contax G mount cannot be adapted to Canon and Nikon DSLRs. However, they can be adapted to most modern mirrorless camera mounts with a relatively thin adapter.

There are several adapters available for Contax G lenses, and they can be categorised into three types:

1. Thin-ring type: generally the cheapest and smallest. I have a no-brand one that is not smooth and often gets stuck thanks to poor tolerances. Trying to focus is a miserable experience. Even those under brand names seem to have the same design (the Kipon one might fare better). I would not recommend one of these unless you can get your hands on one in a store and ensure that it operates smoothly.

2. Thick-ring type: these use a much thicker ring that encircles the base of the lens. The increased size of the ring helps a lot with handling but contributes size and weight. These tend to be smoother and more well made. As far as I know, only the Fotodiox Pro, Metabones and Ulata fall into this category. I use the Ulata myself.

The Ulata Contax G to Fujifilm X adapter

3. Autofocus: Techart Contax G adapter. Only available for E-mount, unlike the others. A motor drives the screw in the lens to enable autofocus, but manual focus is also possible with a small dial on the side.

Image from Techart

For most, the second, thick-ring type is the most desirable kind of adapter. However, there is one more option, that is quite a bit more involved: a lens rehousing. Some folks have managed to rehouse the lens inside the body of another vintage lens, and make use of that lens’s helicoid to achieve manual focus in a fashion similiar to most other manual lenses. You can find more at this thread on Fred Miranda.