A gypsum soil amendment is made by adding gypsum to the soil to improve the texture and to make the soil less compact. It is typically used on soils with high sodium levels.
Soils with high sodium levels are very hard when dry. Gypsum is added to break down the sodium and allow it to leach out, loosening the soil. This type of amendment does not work on heavy clay soils that do not contain much sodium. Hard soil with low sodium should be amended with compost or topsoil. Adding gypsum doesn't raise the pH levels of acidic soil; lime must be added to the soil for pH issues.