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.