First thing first, I'm still not lv 30, so I don't know about ranked play, but here's my take. Whenever I play Galio, I typically build three items every game - Banshee's Veil, Abyssal Scepter, and Frozen Heart. These three give a decent amount of armor and hp, a good amount of MR, and generally allow me to act as a tank, while still giving some "oomph" to my spells.
The way I see it, Galio should be treated as a tank/support hybrid. He can tank, or he can go pure mage, but it's when he dabbles in both that he really shines. Because of his passive, he can do so without losing that much effectiveness. And, with such a setup, you can alter into a more focused build in mid/late game if the need calls for it, grabbing either more tankish items, or more +AP items.
On a final note, I'd recommend always getting at least one decent AP (thru raw +AP or thru high MR) item early, as Galio's two dmg spells can wreck minions and allow you to farm very effectively.