Gypsum (calcium sulfate) adds permanent hardness (calcium ions) to brewing water; 1 gram per gallon adds 62 ppm calcium, 147 ppm sulfate.
Brewers that use distilled, or RO, 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.
Brewing Gypsum is packaged in 1lb powdered bag. Keep away from moisture after opening.