Gypsum and Burton salts are different. Burton water salts actually contain gypsum + potassium chloride & Epsom salts (at least at some HBS's!). To be honest, you really need to know your actual water profile before you start modifying it or you could have unintended consequences. I don't really like recipes that just blindly say "add this salt". What if I live in Burton? What if I live in the Pacific NW?
If you're on city water in the Triangle your water is probably fairly soft and you could add the salts without too much fear, but I really recommend finding out what your water profile is and what the various salts are used for.
As far as the dryhopping I would just add them when you rack to the secondary and you'll get a good two weeks of dryhopping.