For what it's worth : my father had a G1 with the 28 and the 45 lenses. IIRC the 28 hood (GG-1) was 1cm deep and the 45 hood (GG-2) was 1.5cm deep.
Anyway with the 45mm Planar we always found the GG-2 hood to be a bit short (2.5cm in depth would have been better as for sunshading efficiency).
What counts more with third parties hoods
vs. the original Kyocera goodies is the diameter of the hoods. There is a max. which you can't bypass unless you will block the AF beam. Not all third parties 46mm screw-in hoods have the same diameter.
The max. hoods diameter for the Contax G cameras is sometimes said to be 66mm (source :
http://www.contaxg.com/contaxuser/lenses.html#lens_acc)
*but* if you consider that Kyocera made the GG-1, GG-2 and GG-3 hoods to be 57mm in diameter, you rather should get third parties hoods with a diameter inf. or equal to 57mm to be sure not to block the AF beam.