James Clerk Maxwell, in A Treatise on Electricity and Magnetism, states:
"In several parts of this treatise an attempt has been made to explain electromagnetic phenomena by means of mechanical action transmitted from one body to another by means of a medium occupying the space between them. The undulatory theory of light also assumes the existence of a medium. We have now to shew that the properties of the electromagnetic medium are identical with those of the luminiferous medium.
"To fill all space with a new medium whenever any new phenomenon is to be explained is by no means philosophical, but if the study of two different branches of science has independently suggested the idea of a medium, and if the properties which must be attributed to the medium in order to account for electromagnetic phenomena are of the same kind as those which we attribute to the luminiferous medium in order to account for the phenomena of light, the evidence for the physical existence of the medium will be considerably strengthened."
The force of gravity pulls this ether which is composed of infinitesimal particles physically through a given body so as to cause it to flow in a Figure-8 orbital. This, then, is how gravity causes electromagnetism. It is that simple.
Notably, while the Michelson-Morley experiments failed to show the existence of the ether, electromagnetism has been shown to influence the speed of light traveling through a field. See the work of Pelle et al., First observation of magneto-electric directional anisotropy in a gas.
For more information on how gravity causes all observations, see CascadingUniverse.org or read my other posts here on Steemit!
-Steve Scully