I believe it changes the flavor of the plant. I liken it to a burger king tomato vs a home grown tomato. There are differences in plant tastes based on how they are grown. In a given area where farmers bring complex fertility, they'll likely attract the deer more than a plot that is growing but has some sort of nutrient hole in the bucket. There are countless variables that could drive poor plant uptake, but gypsum is cheap enough and safe enough, you can just throw some out there and try it. Maybe don't do all of your acres, but pick one plot, cut it in half, and give one side gypsum, the other none. If that's your issue, the deer will tell you.How does the Gypsum help out?
A complete soil test will also tell you this, and everything else you need to know. When I tested mine, my soils came in at 5 - 8 ppm (my lowest hole in the bucket). Where we applied ammonium sulfate in prior years (to put N on brassicas in fall), we had slightly higher sulfur numbers, but still not adequate. When we put beans on an expanded piece of land that had zero sulfate/sulfur in any form, those beans were the very last of the beans to get browsed. Three other plots within 200 yards of that expansion were completely wiped out before they started on those sulfur deficient beans.