So two part question: how much more gypsum in general to the Amber Full profile for hoppier beer (starting with distilled), and shall I add some gypsum to complete batch/keg, maybe 1/2 tsp at a time, after beer is complete (and would I stir it or just let it settle over time)?
In my opinion, having only 100 ppm sulfate isn't sufficient for any hop focused beers. It just won't dry the beer out enough to let the hops shine. Around 200 ppm sulfate is enough to get the effect. I build my IPAs and PAs with about 300 ppm sulfate.
Figure out if you like the sulfate dose with the pinch in a glass method first. You'll have to stir the beer for a few minutes to get the gypsum to fully dissolve. If one pinch helps, be sure to try another pinch to see if it goes too far for your taste buds.
Once you have an idea of the sulfate level you prefer, figure out a total gypsum dose needed to take your keg from the current sulfate level to the preferred level using Bru'n Water. Then add that dose to the keg, all at once so that you don't admit any more oxygen than necessary. You will have to rock and roll the keg to get the gypsum to dissolve. You'll have an idea of how long it takes for the gypsum to dissolve from your pint of beer experiment.
Remember, sulfate doesn't make your beer bitter. Its makes your beer finish drier.