If you're using mostly extract with a little specialty malt, you generally don't need to add any brewing salts at all such as gypsum -- just use distilled water or soft tap water. I'd skip the gypsum if I were you. It's only for flavor, and in this case, you really don't need it. In an IPA or ESB then it would make more sense.
Like I said before, if you're not mashing base malt (2-row or pale malt or Maris Otter or pilsner malt), then just a simple 5-10 minutes steeping time is plenty. Otherwise, steep any style at 150 F for 30-40 minutes and you should be happy. It really is as simple as that!