No, neither of us would agree with that. Neither the Hulk nor Gladiator have a special resistance to magic.
Superman is not allergic to magic; it doesn't affect him like Kryptonite. He is simply vulnerable to it the way everyone else is. So a "turn into a bunny" spell would turn Superman into a bunny. It would also turn the Hulk or Gladiator into a bunny.
The reason Superman is described as vulnerable to magic is not because he's more vulnerable to it than anyone else, but because he's invulnerable to just about everything else. [Or was, pre-Crisis, when magic, red sun rays and Kryptonite were about it.]
Magic doesn't affect him differently from the way it affects other people -- it affects him the same way it does other people. He has no immunity to it.
And neither do the Hulk and Gladiator.
And no, Thor's hammer doesn't make him stronger, and there are no spells on it that make it hit hard. There are a bunch of magic spells on it, but they've been enumerated, and none of them make it hit harder. The spells would work on Superman, to the extent that they'd work on anyone -- but they'd do what the spells are enchanted to do. They wouldn't do more to Superman than they do to the Hulk or Gladiator, because those two characters are no less vulnerable to magic than Superman.
kdb