Because of the distortion of gravity on space time, the period of a photon is longer the closer you are to the gravity field and therefore the time it takes light to complete a period is longer than for a photon high abovethe gravity field where the period of a photon is smaller and so light takes less time to complete a period. Which means that the clock of a photon's clock above the gravityfield goesfaster than the clock of a photon that is closer to the gravity field.