Don't use the "Mr Color" thinner with the Aqueous Hobby Color paints - it's not compatible. Mr Color thinner is used with Mr Color paints (also by Gunze) which are lacquer based. Don't be confused by the numbers on the jars - these are simply the volume in the jar (eg. 110 250 400 ml) If you want to stick to Gunze's own thinner, use the Aqueous Hobby Color Thinner (wow - that's a real mouthful).
If you are already using Tamiya acrylics, Tamiya's thinner will work fine with Gunze's Aqueous Hobby Color paints. Going further, Isopropyl alcohol and denatured alcohol will also work.
Quality-wise they're comparable to Tamiya paints - they airbrush very nicely, but are a little finnicky to brush paint (just like Tamiya, skinning over quickly and "rolling up"). They are a little thinner in the bottle than Tamiya acrylics, and can be airbrushed straight out of the bottle, but work better with a little thinner. Most of the "flat" colours in the Gunze range are actually low semi-gloss and dry to an eggshell finish.