Michael Faraday is the earliest advocate I have found for the proposal that the final irreducible constituent of all physical reality is the electromagnetic field. He was soundly ridiculed for that as evidenced by the cartoon published in a newspaper of the period. It is only in the present times that we are beginning to see that old Michael was probably right after all. If Michael was right, we should find a gravitational mechanism within electromagnetic fields. We can do that in two ways as follows:
We know that all photons attract each other gravitationally. Gravitational attraction is a phenomena exhibited by all things that possess energy. This phenomena is a well known fact that is proved by the math, but most young physicists come to realize this late. That may be why we have such a problem realizing that all gravitational attraction originates in photons. Since we know that photons create gravity we should immediately suspect that photons are the only source of gravity.
Electric and magnetic activity in an area determine the value of the impedance of space in the area. The impedance of space determines the speed of light through that space. The path of light bends in the direction of increasing impedance. Comprised of light, massive objects must drift toward greater impedance. Magnetic permeability and electric permittivity are the two variables that determine the impedance of space.
If space is empty nothingness, as we suspect it is, space can not warp, bend, or stretch. All observations explained by distortions of empty space must be rethought. This is a wide open field that will yield great discoveries in the future.
The electric and magnetic constants are only constants where electric and magnetic activity are absent. The constants as modified by the presence of fields of electromagnetic activity directly affect light speed. In a gauge field where the value of these increase in one direction, the path of light must bend toward that direction. Any material thing made of light must gravitate toward the increase. That is gravity in a universe made of light.
We know that Planks Constant is a measure of the electromagnetic saturation amplitude of free space. This saturation amplitude seems to vary from place to place in space depending upon the amount of photon fields present there. Photons migrate toward places where photon fields are greatest and the possible saturation amplitude is less. We should be able to find that the value of Planck's constant varies with gravity.
Another advocate of the notion that photons comprise all mass, after Michael, was Albert Einstein. In one of his works linked by the gold button he opined that unification based upon electromagnetic activity should be possible. The fact that this was not pursued, according to Albert, was merely a matter of prejudice.