Anyone remember the Grand Unified Theory super villain team? Graviton (Gravity), ZZaxx (Electro-Magnetism), Quantum (Strong Force), Half-Life (Weak Force). Man they were cool.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
Magnetism doesn't need gravity (as far as I know) but gravity does affect and manipulate electromagnetic fields. While on the other hand electromagnetic fields don't have any direct effect on the force of gravity (they can work against gravity in some ways, but that's different - they don't block and can't manipulate gravity).
Graviton has shown immense gravitational effects including blocking laser light and energy, so I think he could bend and block Mag's attacks. While Magneto could take some measures to protect himself, ultimately his magnetism can't directly do anything to gravity and he won't be able to stop everything. I honestly don't think Mags would even survive the straight "increase local gravity by x-million times" or "gravity crush him into a ball" tricks since a magnetic force field doesn't stop gravity and there is only so much he can do to reinforce a human body against the effect before it would kill him.
Magneto may have some control over gravity, but whatever it is I don't think anyone would argue that Graviton's control over gravity wasn't many leagues above Magneto's.
As far as manipulating electromagnetism on panel, Graviton has deflected lasers, heat, and electricity with his shields and blocked radio wave transmissions with gravity waves. On panel he has also directly dissipated electromagnetic fields using gravity, stating outright that he can disrupt the cohesion of electromagnetism.
I guess that has some loose basis in physics as he was presumably messing with the photons that transmit the electromagnetic force.
But as you say, regardless of real world physics this is comic book physics (at least I think it is). But he is a comic book physicist so he is the expert.
I left out his best feat there the one from the Thunderbolts. Anyway yeah Graviton hasn't exhibited control over the other fundamental forces. He once formed a team of members each able to control a fundamental force.
though, I think both characters (I'm less familiar with Graviton) probably suffer from a lack of imagination with their powers... Someone with the control of gravity that Graviton shows should have little trouble with the avengers...
imho, not for someone who seems to be the embodiment of the entire EM spectrum with some access to other fundamental forces of the universe and low level psi...
that powerset... it makes me huge. Its why mags is such a beast in tourneys
Graviton never did have much trouble with the Avengers, he could just about capture them all with a handwave. But when he was coached to use his imagination and learn finer control of his powers he was pretty much unstoppable. He was able to capture and suspend superhumans from all over the globe at the same time, including guys like Thor, Genis, Hulk, the FF, Iron Man, Jean Grey, Vision etc, while simultaneously levitating every major city on earth, while also beginning to carry out major terraforming operations of the earth's surface - and he did this quite casually and was still carrying out normal conversations. He was able to easily fly to the sun and moon and back. With his knowledge as a comic book physicist he did stuff like the aforementioned instant nullification of electromagnetic fields and blocking radio waves. He could affect and destroy intangible, sonic, and energy beings. He compressed people into tiny balls, tore some apart, and casually threw others into space. He was even able to locate a specific person on the other side of the world and kill them by remote controlling a pebble through their head. It was said that anywhere on earth you ran he could find and kill you. It really was that crazy.
I can fully understand why he has been seemingly depowered by brain damage since then since at that level he was ridiculous and not much use for further stories.
Theoretically, since Electromagnetism is many orders of magnitude stronger of a force than Gravity (but especially on the molecular scale), Magneto should have an edge. But ultimately it's just a quickdraw - whoever has the faster reflexes should get the win.