Huh... I thought Gyro Ball was more Bronzong's thing. Magnezone is... slow but not Bronzong slow (again, its stats across the board are "see Glaceon, maybe add or subtract 5"), it's very very much a special attacker (70 base Atk vs. 130 base SpA) and Gyro Ball is physical, and while Bronzong can make a "be slow, use Gyro Ball" build work thanks to things like Trick Room, Magnezone... you pretty much just want to give it speed EVs, maybe a beneficial nature (if you're not boosting SpA instead,) and just try to fix the problem as much as you can.

Man, looking at Magnezone's moveset (for "does Magnezone even learn Gyro Ball?" purposes, which, yes,) too bad there aren't really good accuracy boosters outside of being a No Guard Machamp or something because a STAB Zap Cannon or Thunder off 130 base SpA would just be rude. Magnet Rise/Thunderbolt/Flash Cannon/Hidden Power (Grass) seems like a good solid (less Zap Cannon numbers but safer) set, though. Charge Beam could be a neat trick to set up for the big sweep while fighting the warmups at the same time, but I don't see what I'd get rid of to make room for it. Like, even Flash Cannon would have its uses against the aforementioned times when some Water trainer is like like "haha surprise" and throws a pure Ice type at you or something.