Simply put, watercolor paints are transparent, gouache paints are opaque. That's it in a nutshell.
Watercolor paints use the white of the "paper" for their white. Lighter gouache paints, even white, may be applied over colored paint under-layers. White gouache may also be added to a hue to make a color tint. Gouache's opaqueness lets me paint in layers from dark to light.
Modernly both products typically have color pigments bound with gum Arabic or similar water-soluble binder. Gum Arabic is a natural, non-toxic, weak binder. Both paint types have a little preservative and plasticizer to extend their shelf life.
While a form of gouache was developed during the middle ages, today's designers' gouache was developed for professional illustrators.
Student gouaches (and watercolors too) are applied like designer gouache. But student gouache, like Reeves and Artist Loft, has less pigment to binder, more filler, odd color options, and fewer choices. The manufacturers use synthetic hues for traditional single pigment colors like Ultramarine blue. They are economical but less pure or permanent.
There is also acrylic gouache. Like traditional gouache, it dries to a matte finish and is opaque. Its acrylic binder makes it unlike medieval manuscript paint, but resistant to water when dry. It's useful on more surface types, such as wood, fabric, and metal.
It is possible to use watercolor paint for illumination if you use it with white gouache. The technique is not like watercolor painting. If you already have useful artist quality watercolor paints, buy some white gouache to get started. You may like it.
That's the difference between the various gouache types and watercolor. They each have their use and benefits.