hydration is very exothermic and also occurs very fast in the fresh concrete mix as the temperature quickly increases with the progress of the hydration reaction. The effect of gypsum addition is to promote the formation of a thin impervious film of ettringite at the surface of the  grains, passivating their surface, and so slowing down their hydration.
The hydration process in fresh concrete is accelerated by its exothermic nature and rapid temperature increase. Gypsum addition helps to slow down this process by forming a protective layer of ettringite on the surface of the concrete grains, which acts as a barrier against further hydration.