Gouache is a medium that goes back a long way. The watercolors that were painted by Durer were actually done in Gouache which is opaque watercolor. The medium was used to illustrate manuscripts in the middle ages. It has been used by just about any artist you can mention, François Boucher, JMW Turner, George Innes, Winslow Homer, Edward Hopper, etc. When you hear the term body color (which was used in a great deal of studies by the painter Peter Paul Rubens) it is actually gouache the artist used.
I like the medium for several reasons.
It is more forgiving than transparent water color. I also love working in transparent watercolor but sometimes I would rather work with the advantage of building the image without worrying about leaving areas white and dreading that irreversible mistake.
Because of its opacity I can build the image in the same way I would paint in oil but without the solvents and without the long drying time.
It's an easy medium to travel with. The set up would be the same as transparent watercolor.
You can read up on the history of the medium by clicking here
There are some great examples of Gouache paintings at this link
A lot of golden age American Illustrators also used gouache. It would be a good idea to research those illustrations to examine how they used the medium and what is possible with it. Steven Dohanos and Harry Anderson are two illustrators who used the medium often.