Calcium chloride is usually used in place of Gypsum when sulfates are not desired. Calcium chloride will adjust your mash pH down. 1 gram in 1 gallon changes the salt levels by 72 ppm calcium, 127.5 ppm chloride and adds 180 ppm to the hardness.
This is food grade anhydrous Calcium Chloride. Sold by the ounce (28.3g)