What is gouache?
The technical definition is apparently "A method of painting with opaque watercolors mixed with a preparation of gum
Gouache is like working with watercolors but being able to go back in and work with a piece hours later, like oils. The colors are vibrant, which reminds me of acrylics.
The paint is expensive, but a little tube can last a very long time if one is using them more like watercolors.
So, it isn't automatically transparent and can be used without thinning it out. An artist can choose the opaqueness of the paint pretty easily. Sometimes the colors lighten when they dry and then can be layered or not in stages. This can be a much thicker paint than other types of paint.
Gouache is what artist Alex Ross often used for his famous superhero portraits.
I'm not as good with gouache as Alex Ross.
My dancing girls are done in gouache:
To compare, here are a couple things done in watercolor:
I have no idea if I answered your question adequately.