Is it the measure of it’s energy?
I think it is that photons have ‘momentum’. and gravity bends the momentum.
So while a photo has no mass, gravity bends its because of its momentum.
Einstein put it as space-time is warped. So light remains straight, it is just on curvy terrain …
Unless all our physics are BS,
Gravity affects every thing that has mass.
Note this Gravity doesn’t effect my thoughts
or beliefs.