While both mediums use water-soluble pigments, there are characteristics and methods of application that make them different.
Gouache is an opaque watercolor. There's a high concentration of original pigment with a white pigment added (such as chalk) giving it a very vibrant and solid color. This allows artists to paint in layers, covering previous layers easily. Working in detail can be easier in gouache as it allows for broader and more expressive strokes.
Watercolor is more transparent.Pigments are mixed with water and then applied to paper. The primary characteristic of a watercolor painting is its translucency, allowing the base color of the paper to shine through, giving a luminous and delicate appearance. Watercolor allows for subtle blending of color so is often used for delicate and flowing artworks.
Watercolor is done on exclusively on watercolor paper, whereas gouache can be used on illustration board or mixed-media surfaces like canvas.
Teri Gammalo, whose images are above, enjoys both.Here's what she has to say."Each technique offers unique aspects.I like the spontaneity of gouache as it is easier to use and requires less colors. It is better to use on location, where I can take a few paints, a pot of water and create an image. It is a good tool to use for study drawings. Sometimes I'll come back to my studio and create the same image using watercolor or a combination of both.”