As I had never tried gouache before I was curious to know why you would use it. It appears to be a fast drying water based paint with lights drying lighter and darks drying darker. Apparently it lends itself to wet into wet techniques and does not require a keep wet palette as is remains workable for ages. That is also why it desperately needs to be varnished.
There is an interesting article about painting with gouache by Ralph Parker who tells us about various aspects of the paint and why he is so fond of it.
Ken Bromley Art Supplies also have something to offer on the subject.
Here is a video to show you how it is applied.