Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
Offense
+5 AD or 9 AP
Flex
+8 Magic Resist
Defense
+8 Magic Resist
WhyAftershock
Aftershock is the successor to Courage of the Colossus, but rather than granting a shield it does damage and give a ton of Armor and Magic Resist. Galio loves Magic Resist
WhyShield Bash
Shield Bash synergizes well with Galio's [W] shield, granting a bit of extra resists as well as offensive power.
WhyConditioning
The extra resists from Conditioning empower your ability to absorb a ton of damage in the mid game with [W] and also grant additional damage reduction on your [R].
WhyOvergrowth
Overgrowth on Galio makes you a late game monster, empowering your health pool to be a huge frontline tank that dishes out a lot of damage.
WhyManaflow Band
Galio's abilities have relative high mana costs and be needs to spam to keep up with wave pushes. Manaflow Band will give you the needed mana sustain to shove and roam.
WhyTranscendence
Entering the mid-game, Multiple successful taunts from [W] will make or break your teamfights and CDR is what enables that.