Calcium Sulphate is an important mineral for its effect on mash and wort pH. Used to harden water when brewing ales and bitters. Gypsum adds permanent hardness (calcium ions) to soft brewing water. Brewers who use distilled or RO water often add some gypsum to the boil to give it some hardness.
Gypsum can also be used to lower the pH of the mash.
https://ezwatercalculator.com/ for help calculating water chemistry.