Gypsum 2 oz
Gyspum (CaSO4) does a couple different things. If you add it to the mash, it can lower the pH. The added sulfate content will also accentuate the bitterness of your beer. If you want to increase the sulfate for bitterness enhancement but don't want to change your mash pH, you can add it directly to the kettle.
Always know what your beginning water is before adding anything, otherwise you won't really know what your levels are.
As a general rule, use to add permanent hardness (Calcium ions).1 gram per 1 gallon changes the salt levels by 61.5 ppm calcium, 147.5 ppm sulfate and adds 153.5 ppm to your hardness.