Sorry, but the remote electric shutter release for the Contax G2 will not allow you to depress it half way for focussing only. It will only go all they way so it will focus and immediately take the picture. It also has a lock on it to hold the shutter open for Bulb "B" exposures. You could manually focus the camera first (MF mode), if that is an option for you.
Great camera with great lenses, if you can find a way to get the set-up to work for you.