Gypsum adds hardness to soft brewing water. Brewers that use distilled water like to add some gypsum back into the water to give it some hardness.
Gypsum can also be used to lower the pH of water. Use 1 teaspoon per 5 gallons.
Gypsum (also known as Calcium Sulfate) is used to add permanent hardness (Calcium ions) to brewing water.
1 gram in 1 gallon changes the salt levels by 61.5 ppm calcium, 147.5 ppm sulfate and adds 153.5 ppm to the hardness.