>>>PE = mgh (I know this, because those are my initials)
>>>
>>>So when we solve for a photon, which has a mass (m) of zero, the result is no gain in potential energy.
>>
>>A photon does not have a mass zero. Since it has energy, it must also have mass. If physisists say that it has a rest mass of zero, this simply means that a photon moves at the speed of light. The "rest mass" of any particle moving at the speed of light can't be different from zero, or else, at the speed of light, it would have an infinite mass.
>
>Rest mass = mass:

No, the rest mass is the mass, assuming that the object is at rest.

For instance, an object at rest may have a mass of 1 kg. After accelerating it close to the speed of light, the mass might increase to 2 kg. For all practical purposes, the 2 kg. are, at that moment, the real mass. Energy has been invested to increase the speed of the object; the energy is now part of the moving object, as kinetic energy.

You have to distinguish whether you mean the "rest mass" or the "moving mass". Sure, in the case of the photon, if by "mass" you mean the "rest mass", then, indeed, it has no mass. But the rest mass is the mass under the hypothetical assumption that the photon is AT REST. Real photons are never at rest. As to the mass while it is moving: since the photon has energy, it therefore also has mass (E = m * c^2, or m = E / c^2).

>Most physicists would disagree with you that a photon has mass; and most would disagree that any object feels "gravitational attraction" because in general relativity gravity is not an attractive force, it is the geometry of space-time.

It doesn't matter; the point is, the photon, having mass, is no different, with respect to gravitation, than any other particle that has a mass.