Galio's special little thing is that for every point of magic resist he has, he gets half a point of AP.
All of his abilities are modified by his AP score to some degree or another.
His ult is the swirl that taunts everyone in its area, then explodes to reveal X amount of damage, plus AP modifier, plus up to 40% of all the damage he took during the taunt (he only takes half damage during the taunt, though).
Our success with him in pvp has been bad more than good, but I am still new with him, and am learning. I just played a PvAI game, and twice, with a combination of teleport/ult, and Flash/ult, I was able to get into the middle of 3 enemy and ult 2 of them dead.
Now, real people deal with his ult better, and I know at least Alistar can knock him out of it. Also, the other random players I was with were better at, "Oh, Galio is gonna ult, let's get in on this."
I am also getting better with the buff I can lay on peeps.