How is a light photon affected by Gravity? Without invoking General Theory of Relativity, we can use an alternate method to see how the energy of a light photon is affected by gravitational field. Since light has Energy, by virtue of Mass-Energy Equivalence Principle, therefore light has ‘mass’ due to its Energy. This mass can experience gravity.

When such a photon is moving upwards, it will loose Energy and thus its frequency will reduce.
When light escapes the gravitational pull of a massive star, its frequency is considerably reduced (for visible radiation, it’s shifted towards the red end of the spectrum). This phenomenon is known as Gravitational redshift.