Gouache is a watercolor paint and much of what I say here can apply to ordinary watercolors, but gouache is generally more opaque than typical watercolors. 
This means that it has the properties of watercolor (it doesn't dry up permanently and will continue to be active every time water is applied.) 
It also means that it has the intensity and opacity of acrylic, producing art that looks similar in some ways.
Yet it is a form of watercolor and thus water-soluble.
Gouache is often ignored as a painting medium, but like acrylic it can be used to achieve a wide range of effects. Gouache is best described as semi-opaque, and I think it's best in many cases if dark tones are layered over lighter ones. 
Keep in mind that you can't use the acrylic method of 'letting one layer dry' before applying the next. That's the frustrating part about any watercolor medium. Once a color is there, it can't be fully removed. Try layering another on top of it and the two will invariably mix to some degree. 
This means a gouache work generally needs to be planned out ahead of time.
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