Gypsum (calcium sulfate) is a brewing salt added to water specifically to boost calcium and sulfate together — the single most common salt addition for chasing a sharp, dry, hop-accentuating character, and the direct modern descendant of "Burtonisation," the practice of adding gypsum to replicate Burton-on-Trent's naturally sulfate-heavy water anywhere in the world.
Two separate effects from one addition
Gypsum's calcium contribution helps buffer mash pH and supports healthy yeast function, entirely separate from its sulfate contribution, which is the part actually responsible for the sharper, drier perceived bitterness brewers reach for it to achieve. A brewer wanting the calcium benefit without pushing sulfate as high has to look toward calcium chloride instead — gypsum can't deliver one effect without the other.
Easy to overdo on a beer that isn't built to carry it
Pushing sulfate very high reads as harsh or minerally rather than pleasantly crisp if the beer underneath isn't built with enough malt and hop character to support it — genuine Burton-level sulfate (Burton-on-Trent's historic water runs 820 ppm) is far beyond what most modern hop-forward American styles actually target; a smaller, more restrained gypsum addition is the standard practical approach for most homebrew recipes rather than replicating that extreme exactly.