It's thematic and kind of fun. But I think there are better ways to maintain his identity as a magic tank.
It doesn't actually make him any better at tanking magic damage, it just rewards him with damage for building MR. At the same time, it's kind of a trap, because a tank like Galio doesn't really need that much MR to be able to survive a lot of magic punishment. Building more than two big MR items is a waste of gold, but his passive doesn't ultimately feel that impactful unless you stack a totally unnecessary amount of MR.
It's also kind of strange given his ult. It feels like a waste of his kit to build early armor. But, at the same time, he has relatively low base health, and his ult is an AoE taunt.
Basically, his passive is fun, and strong in very specific situations. But in most games, it's hard to make a lot of use of it. In most games, his passive basically just gives him 70ish free AP in the late game. It sounds like a lot, but his abilities have poor ratios.
Honestly, his passive could be totally removed, and his base damages and AP ratios could be tweaked, and he wouldn't really play all the differently. He would be slightly better at fighting AD champs... and that's about it.
EDIT: In my opinion, he should gain a modest MR ratio on his Q, and his passive should be changed to return a percentage of his mana when he takes magic damage. It would maintain his niche as a magic tank, while solving the main issue that I think prevents keeps Galio from seeing play--his crippling mana costs that make him incredibly weak in lane early on, even against champions he should be strong against.