I have a couple bottles of Gunze Sangyo, I don't know if they're better or worse, just different. I think they might be a little thicker than Vallejo, out of the jar, but then again, they are formulated like Tamiya paints, if I'm not mistaken, that is, they are designed for scale modelers and for airbrushing. So, they're intended to be thinned out a little more, or differently, perhaps, from the Vallejo colors, which are really designed for brush painting on figures.
Having said that, GS has a huge palette of colors, and you might find colors there that you can't find in other makers' catalogs, and so, you use them and adapt as necessary. I know that their Orange Yellow is, right out of the bottle, spot on for the US Navy's yellow for the upper wings in the 1930's color scheme. No one else makes a color that comes close, so you have to mix your own.
I use isopropyl to thin them, but added right into the jar and stirred in.
Hope that helps, prosit!
Brad