@Johnny-mariseed This kind of depends on the make up of your soil, the pH and whether you water to run off. The gypsum will dissolve and the calcium becomes freed up for the plant, while the sulfate (-- anion) will usually bond to another (++ ion) like magnesium and create epsom salt (magnesium sulfate) which will then leach and form crust on your bag. If you have high amounts of salts though, I think it would prefer to attach to sodium (++) and then can be leached out in the same way. If none are present, it could rob your Potassium however, but most soils have enough mag to not have to worry about this.
I topdress once the plant gets about a foot high and don't reapply.
I have finally given too much sulfur to my plant, but it only lasted until the plant got to about 24 inches and I saw no signs of the excess. (burned copper teeth on leaves margins that I thought was K)