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Be aware that traditional gouache is not water-fast, and underlying layers will be reactivated by the moisture in any over-painting.
Ok real gouache or tempera as its often called, is an opaque watercolor. It used to be very popular among illustrators and designers in the 50's and 60's. In fact most of Angus McBrides work is painted with gouache. As others said its not water resistant, even if its completely try you can activate it with water.What you want is acrylic gouache which is standard acrylic paint mimicking the matte finish of real gouache. I know some people that use the Jo Sonja brand with great success. Its a finer quality craft acrylic in tubes.