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How much better does the Gok20k shave in comparison the Naniwa 12k?

About $150 better.

If you actually believe that no one uses a Naniwa 12k for long then there should be plenty of used ones for sale. Yet how often do they come up?

Woodcraft is a good place to get them. However, as a natural stone, they have no real grit designation and some will, and some will not, improve an 8k edge.