One of the strongest mid atm, great laning sustain, and great ultimate. can carry a whole team if used ultimate correctly.

pros:
- great ultimate
- pefect for mid lane cuz of all you mr against a mid champ
- great damage, tanky.
- awesome passive

cons:
- can get countered by ad mid mike talon, cuz the mr doesnt do anything on ad
- if somebody has next attack stun like udyr, then the ultimate wont do much.
- melee range
- early game farming is a bit hard cuz of your melee range

for runes i take my normal mid runes, for ap and magic resist, now you got great lane sustain and good damage.

for masteries i take 20/3/7, now you got your ap, and the magic resist (wich is ofcourse needed on galio), and your mana regen.

for items, i start with boots and 3 health pots (if you think you are gonna have it hard then you can also start with null magic mantle).
Then your chalice of harmony, for a bit of ap, and you cant akmost go out of mana
Then an abyssal scepter (because you have to get in close range for the enemy team with your ultimate),
Then rush crystal scepter, for health and ap,
Then your zhonya's for the awesome active,
Then finish your chalice of harmony to your athene's unholy grail,
and end with GA. you got lots of damage with this build and you are supertanky from it.