Without the addition of gypsum, clinkers of cement when grinded does not have the property called initial and final setting time.
All it have is quickly setting property as soon as it comes in contact with water.
Gypsum modifies this property and allow us to get minimum 30 minute time to use it after addition of water and ensure it sets within max 10 hours time.
Gypsum dosing if goes wrong can lead to following.
Flash Set of Cement
False Set of Cement.