[zaarn]

Easy way to check, just wave a long piece of iron through the air. If there is truly a massive EM field, it should induce a decent voltage into said iron rod as it moves through the magnetic field lines produced by the "gravity EMism"

Considering noone died from waving iron rods through the air by way of electrocution (other than lightning) I'd say that means gravity is not an EM field.

There is also a different test: EM fields are either positive or negative in charge. If you have three objects, in an EM field they cannot all three attract eachother, one object must be pushed away from one of the other two.

Considering we don't see that behaviour in our solar system, gravity cannot be an EM field.

[100ideas]

I like your elegant thought experiment!

Quarks have -1/3 or +2/3 charge in the standard model and presumably(?) at high enough energies they separate out into a quark-gluon plasma thus exposing the universe to their unholy fractional charges. Above 2 trillion Kelvin according to wikipedia.

[zaarn]

Even with fraction, either you have repulsion or attraction, depending on the sign. -1/3 charge will be repulsed by +1 or +2/3 charge, just at different strengths.