Adding Gypsum to your soil can be beneficial, but first you need to understand what type of soil and soil structure you have.
Clay soils are also called "heavy soils." Clay soils are composed of flattened, platelike, microscopic particles that pack closely together, leaving little pore space for either water or air.
Clay soils can hold great volumes of nutrients in a soluble form when broken down. They also hold water for a longer time when wet. Drainage is slow in clay soils, so the loss of soluble nutrients through leaching is also slow.
Sand particles are comparatively large, and they are irregularly rounded rather then flattened. Their size and shape allow for much larger pore spaces between particles than in clay soils. Therefore, sandy soils contain lots of air and drain well.
In sandy soil, the surface area of the particles is less than in the same volume of clay, so the volume of soluble nutrients in sandy soil is correspondingly lower.
Loam is the ideal garden soil. It contains a mix of all three particle types - clay, silt and sand. With a combination of large and small pore spaces, it drains well but does not dry out too fast, loses nutrients only at a moderate rate and contains air.
Soil texture is defined by the size of its primary particles; its structure is determined by the way those particles bind together to form small clumps, called aggregates. Soils also contain cations and ions, similar to positive and negative charges. Soil aggregates are held together by cations (positive charges), and soil particles are negatively charged so they repel each other. The presence of sodium (salt) inhibits the cations from forming aggregates which in turn creates poor soil structure.
Gypsum (calcium sulfate) contains calcium, hydrogen, oxygen and sulfur.
A mineral, calcium sulfate (gypsum) is used to add calcium to the soil or to improve the structure of clay soils without affecting the pH. This amendment is primarily useful for improving sodic (alkali) soils. Our soils contain an excess of sodium and an excess of calcium. However, the calcium is not available to plants because of high pH from calcium carbonate (caliche). In a chemical process similar to what happens in a water softener, the calcium from the gypsum displaces the sodium from the clay particles (poor soil structure), and the sodium washes through the soil profile with rain or good irrigation practices.
Gypsum is a neutral product which does not increase or decrease the soil's alkalinity. Unfortunately, since the Gypsum does not change the pH level in our soils, it is of no benefit in releasing calcium to our plants. But Gypsum has been shown to improve water penetration.
Let's review that: Gypsum contains calcium which we already have excess of, and gypsum does not reduce pH nor disperse salt. However, it may be beneficial to use in clay soils to aid in water penetration.
In sandy soil, the use of gypsum will be to no advantage, since the salt is not bound to the soil aggregates and cannot lower pH. Much the same is true for loam, where adequate water movement is not inhibited.
Other materials such as soil sulfur or sulfuric acid can be used to reclaim sodic soil. Soil sulfur is slower acting than either gypsum or sulfuric acid.
The beneficial effects of organic matter (humus) in the improvement or maintenance of soil properties has long been known. Soil organic matter serves as a reservoir for nutrients, improves soil structure, drainage, aeration, cation exchange capacity, water holding capacity, and provides a source of food for microorganisms.
Generally speaking, soils with high organic matter have improved soil structure as compared to soils lower in organic matter. The use of gypsum is not a cure-all as some people would like to believe.
In the long run, the best thing you can do to improve your soil and keep it healthy is compost.
Be good to your soil and it will be good to you.