The establishment (most turfgrass researchers, teachers, extension agents, and others in the trade for many years) do not now accept the use of gypsum for soil ammendment or conditioning.
If you want a sellers' viewpoint, go to http://www.awgypsum.com/
. There are links there to other info.
One of the people who really got me going in understanding turf, a teacher now long retired, has been trying to convince me to try gypsum for 3 years. When we went out on my properties this summer, he did convince me to try it next year. I'll be setting up the testing with him later this winter, so I'm not now really up on the details. I believe he uses 2-3# per 1000 ft² per application. And I do not know about the frequency of application, or the longevity. It seems from the little we have discussed that he has accomplished long term success with initial applications.
On one of his smaller projects, a woman wanted to plant a vegetable garden in an area of lawn with a hardpan; Mr. D. just added gypsum in the garden area, and she had a successful garden with no hardpan - while the surrounding area still retained the hardpan. His biggest was an old landfill, a mountain of trash capped with clay. Codes required plantings to stabilize the hill, but nothing would grow in the clay, so the owner capped the clay with 6" of topsoil. But before he could get anything to grow on the hill, the topsoil would slide down to the bottom. 3 years ago, Mr. D. had the hill treated with gypsum, and the problem was eliminated. He is now looking for a large farm to try it on farm fields. And his gypsum comes from recycled sheetrock; he's working with the recycler to see if the successes can be duplicated.
Anyone want to try this in their area, email me after mid-Feb, and I will have more details. It's not gonna be a recipe, more like research. But I'm sure Mr. D. will help us in setting up proper testing.