Have you brewed this particular IPA recipe before? If not, you should probably brew it first using your regular dechlorinated water to get a baseline for comparison. Keep a couple of bottles back so when you brew with modified water you can compare the difference. Too much gypsum in a hoppy beer can impart a distinctly unpleasant lingering harsh hop bitterness, so be careful as you go. A "Burtoned to death" IPA is not a pretty thing.
It's also my experience that water is an "advanced setting." As long as your water is good to brew with, until you've mastered technique, yeast management and recipe design, you really won't see the benefits of water adjustment unless you're trying to brew something like a Bohemian Pilsner. If you're tweaking your recipe for the sake of tweaking it, play with hop and malt varieties and mash temperature first. Then, once you've gotten pretty close to what you want and can brew it consistently, start messing with water adjustment.
I also agree with a10t2. The numbers for "city water" are actually a range not some magic single number. For river water, and some springs, mineral content is heavily dependent on seasonal rainfall. For well water, wells sunk to different depths might draw from different aquifers, and might have different mineral contents, even for breweries which are relatively close together. Cities like Burton and Edinburgh are notorious that way. Finally, brewers have been treating their water since the late 19th century and are often coy about telling outsiders how it gets treated. What comes out of the brewery taps isn't always what goes into the mash tun.