Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Do not build Galio around his passive, you will fail, just consider it a nice bonus. His shield gives him plenty of armor and MR, so one item of each should be more than enough.
Remember you can break his ulti early by moving, same as Nunu, but yours does a lot more damage even when broken. If the enemy team is stacking Mercury's, you will want to do this.