The stones are now en route to Finland.
Here are my impressions:
Overall, I really liked all the stones a lot, they give great feedback, are a joy to sharpen on and are fast. They are also beautiful stones, especially the 6000 and the 300. My whole family was impressed by the looks of these stones.
I've tried different kinds of steel on the Gesshin 300 and it always worked very well. My King Deluxe 300 was pretty ineffective for stainless, not so the Gesshin, whatever I threw at it, it had no problems removing steel. It does dish pretty fast and the stone was pretty thin when I received it.
The Gesshin 2000 is a great stone and one I would like to own. It seemed that it can do whatever a 1000 can do but leaves a finer edge and could be also a good one-stop for regular stainless knives. It does clog up a bit but can easily be cleaned with a diamond plate.
The Gesshin 4000 is another great stone, it still feels pretty coarse and is fast. I guess it could be used as a finisher for stainless or coarser finishes or as part of larger progression or for touch-ups.
The Gesshin 6000 was my favourite in terms of looks, it has a beautiful colourful pattern, it feels very smooth and is a joy to sharpen on, although the JNS 6000 seems a bit faster to me.
Overall, I really liked all of them and would be happy to have them in my arsenal.