You might try the method advised by Murphy's in the link I gave earlier. Some waters dissolve gypsum more easily than others but many people experience the same as you. Years ago some major brewers transported water from Burton to their breweries.
Gypsum doesn't dissolve very well in my water either. That comes mostly with 135mg/l sulphate from gypsum deposits within the dolomite from where it is extracted, but when it rains it is quickly diluted. It's possible it took 25 years seeping through those rocks to reach that sulphate level from gypsum.
My water also has 255mg/l of alkalinity as CaCO3 which when treated with sulphuric acid adds another 225mg/l sulphate with absolutely no fear of any deposit of this sulphate salt, yet I would struggle to get any gypsum to go into solution. Gypsum additions are then made to the mash, sparge and boil to achieve sulphate levels of 400mg/l and above.
This doesn't help you with your problem and I think you will, just as I did, have to find your own way with your own water or take a trip to Burton. There are many, perhaps even a majority presently in this American dominated beer world who would think your water is better for making beer than mine, but both can be made to work with effort.