Here's why:

* Calcium: The cation present in gypsum is calcium (Ca²⁺).

* Sulfate: The anion present is sulfate (SO₄²⁻).

* Dihydrate: The formula for gypsum includes two water molecules (H₂O) per formula unit, making it a dihydrate.

Therefore, the systematic name combines the names of the cation, anion, and the number of water molecules.