Gypsum is not used to reduce soil pH because it will displace the soil H+ but there's no means of extracting the H from the soil. So the pH of the soil remains the same.
44.8% oxygen, 21.5% silicon, and 22.8% magnesium. There's also iron, aluminum, calcium, sodium, and potassium. These elements are all bound together in the form of silicate rocks, all of which take the form of oxides.