It is not possible to focus a Contax G lens on the lens in the conventional way. There is no focusing ring. The Contax G lenses were designed as AF only. The only way to focus is to turn the focusing screw on the back of the lens - normally done by the AF mechanism of the camera.
Thus, all NEX - Contax G adapters have to use some form of a focusing ring (or similar) built into the adapter to focus the lens. This may seem awkward, but in practice it is alright. My hand naturally wants to be slightly further forward, but it's OK really.
You've got to bear in mind that the Contax lenses were never designed for this purpose, whilst Leica and Voigtlander lenses were made in a more conventional way and so adapt more easily to being used on a NEX.
I chose Contax G simply because they are relatively cheap and superb quality compared to the equally superb , but much more expensive Leica optics.
As to adapters, I got a no-name Chinese adapter off Ebay which has proved quite satisfactory although a friends one wasn't quite so good. The contax lenses are a bit fiddly to mount on the adapter (don't give up straight away), so practice until you get the technique!