Ideally, a brewer needs to know what the ionic content of the extract and brewing water is before deciding on adding more minerals. In some cases, finding the extract ionic content and its result in your beer can be a little tough. Finding out the ionic content of your water should be a little easier, especially if you start with RO or distilled water. In many cases, using RO or distilled water is safest when brewing with extract. That way you don't have to worry about overdosing any particular ions.
From my review of the major US extract producers, you wouldn't have to worry about excessive sulfate in their extracts. For Briess extract, you do have to worry about elevated sodium so don't add any sodium or use a water with much sodium.
So with those caveats, there is no reason not to plan on modest sulfate additions to improve the perceptions of this planned hoppy beer. The sulfate will help dry the finish and improve the perception of hops and bitterness in the beer. I would use a tool like Bru'n Water to help calculate how much sulfate-containing mineral you add. I wouldn't increase the sulfate concentration any more than about 250 ppm since there is some sulfate in those extracts. If you are not a fan of dryer and hoppier ales, then I would further reduce that sulfate target. Its all a matter of taste.
Add the gypsum to the boil. It does not matter much when, but I would add it early so that there is more opportunity to fully dissolve.