Forget the gypsum, you are brewing an extract beer. Your water chemistry is already "locked in" to the extract, at the malt house when they made it.
From "How to brew,"
... salts are more properly used to adjust the pH of the mash for all-grain brewing. Water chemistry is fairly complex and adding salts is usually not necessary for extract brewing. Most municipal water is fine for brewing with extract and does not need adjustment. So, if you are brewing from an extract recipe that calls for the addition of gypsum or Burton salts, do not add it. The proper amount of a salt to add to your water depends on the mineral amounts already present and the brewer who published the recipe probably had entirely different water than you do. You may end up ruining the taste of the beer by adding too much. Just leave it out; you probably won't miss it.
Take a look at my "Old Bog Road Brown Ale." A lot of people like it. http://www.homebrewtalk.com/f66/old-...w-grain-61591/
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