I'm brewing from an Austin Homebrew extract kit, and it came with some gypsum. I thought gypsum was only for modifying water chemistry, but there seems to be some other schools of thought around.
So:
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. The best way to decide how much to add is to get a water analysis so you know what your baseline is and how much gypsum you might need. You should never add water treatments just because a recipe says so. Know your water, then you'll know if you need them and how much you need.
Stouts dont need water adjustments, the higher the pH the better - dark malts absorb alkalinity and levels the pH automatically.
Austin Homebrew's kits are top notch as is their support. I strongly suggest you contact them if you have any questions regarding one of their kits. If they added it, it wasn't done arbitrarily. It may be a particular style of beer may benefit from it, such as a stout.
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