If you mean you can’t use white to essentially erase other colors, then check Brush: Alpha and Texture: Polypaint Mode to make sure it isn’t set to 5 (Darken). This setting won’t paint white since white isn’t darker than any other color.
If you mean you want to paint a pure white color and have it show on top of the gorilla matcap, that won’t happen due to how materials work inside zbrush. What we see is a mathematical blend between the surface colors derived from material and the polypaint (vertex color) data. As a result, white polypaint will basically just mean we see the pure material information, so if you want white paint to appear white then the material itself needs to be white.
If the latter is the case, you might be better off taking a white matcap or basic material like skinshade, and using a dark grey polypaint to darken it towards an appearance similar to the gorilla material.