The difference between gouache and watercolour whites is the amount of pigment in the paint. Gouache is more opaque, which is why it is used for block colour.
The two mediums are intermixable. Gouache and watercolour can be used together when painting in layers, or the two can be mixed to achieve varying degrees of opacity.
Because of their opacity, Designers Gouache whites are frequently used to create highlights on watercolour paintings.