Gypsum is added to cement to:

Concept: 

Clinkers are created during the cement manufacturing process; these clinkers are then cooled down and combined with a small amount of gypsum before going through the final grinding stage.

Gypsum is added during this manufacturing process to slow down the hydration process of cement after it is mixed with water. Normally, when water is added to cement, it begins reacting with tri-calcium aluminates and gets hardened; however, when gypsum is added, the reaction with $C_{3}A$particles takes place to form hydrous calcium aluminium sulphate, resulting in the formation of very fine cement particles, which forms a coating on $C_{3}A$'s surface. 

These crystals are too small to fill the spaces between the cement particles, which slows down the cement's initial setting when water is added. If water is not added, however, the cement sets rapidly, leaving no time for the placement of concrete, thus the cement stays pliable and usable.