First, only ringbearers can even see the rings of power, so no Narya for Magneto. Grlamdring, however, would get squished.
Gandalf can always explode Magneto, if need be. However, it’s much more likely he’d inspire Congress to declare mutants enemies of the state and touch off a war of epic proportions, rather than do anything useful himself.
If it were a typical comic book fight, I’d say the battle would go something like this:
Magneto would get Gandalf tied up in some sort of metal cord/wiring/piping/whatever, and begin to gloat. Gandalf, in the meantime, would summon wolves, or some other animal to sneak up and attack Magneto, distracting him enough to allow Gandalf’s escape. Then, while Mags is busy, G would pick up his stalf, walk on over, and knock him on his head, put him down, and move on.
If the two of them really wanted to hurt one another, they’d have to put in a lot of concentration, leading to a bout where just as Magneto rips Gandalf in half, Gandalf makes Magento explode, and then they’re both dead.
Of course, apparently Magneto just tore apart New York City, killing hundreds with barely a thought, so I’m thinking Magneto could pull it off without a thought. But Gandalf has that nasty habit of coming back to life more powerful, so…it all depends on what environment they’re in, I guess.
Ah, but just because Magneto can’t see Gandalf’s Elvish ring, does not mean that he can’t sense its presence magnetically – or, for that matter, yank it off his finger magnetically.
So that’s what happened after the End of the Third Age! Gandalf became Senator Kelly!
Magneto seems to have a very similar style to Saruman… The same sort of emphasis on technology and machines, and they both have this thing about being superior to all others, and being eager to prove it. So I suspect that Magneto could beat the Grey, but that the White could beat Magneto.
Mags could definitely fling Glamdring around, though probably not damage it. While not exactly adamantium, that sword is tough (remember that the Balrog’s sword shattered on first contact with Glamdring). Narya, I’m less sure. The thing with not seeing the Rings isn’t actually a matter of vision, per se… I don’t think that a non-Ringbearer is capable of being aware of a worn Ring of Power unless the wearer wills it. But then again, an anti-telepathy helmet might offset that. There’s also the question of whether Magneto can manipulate all metals, or only ferromagnetic ones (which, as I understand it, varies based on who’s writing him). I’m not sure what Narya is made of, but it’s probably gold, silver, or platinum, and certainly not iron.
But even if Magneto can disarm Gandalf of both his sword and ring, he still has his wisdom, his staff, and his own native power (or that portion of it that he’s allowed to use).
Of course, Gandalf’s power isn’t really in power, and he avoids head-on confrontations whenever possible. He’d be much more likely to be leading a group of other heroes than directly confronting Magneto… In much the same way that Xavier does.