Gypsum is recommended for Sodic soils and not for saline soils. Gypsum is Calcum Sulphate, so it adds both Calcium and Sulphate in the soil. Sodic soils are low in Calcium and thus its addition helps in renovation of such soils.

Also, Sulphate part of Gypsum is converted to Sulphuric Acid in the soil which lowers soil pH. as sodic soils have high pH. For saline soils, leaching of surface soil by flooding the field is recommended. Also, selection of proper crops and cropping pattern are recommended for these soils.

Source: Study and Research Materials on Soil Science at Punjab Agricultural University