_Galio is week until 6, you need to be ***** 'til the R.
_Level up your shield, because the more important thing of your Q and yor E is the buff of speed movement, and the control of the Q.
_Level 6, you became Undivable, and If you've got your Flash, you're really usefull for a gank. You can even go gank yourself the midlane, or your own lane.
_In midgame, use your Flash-R on the AD champ.
_In Lategame, you can Flash-R undertowert, IF you're tanky enough. The damages of the tower Increase you're own damages.
_In lategame, you can Use you're W against the tower In the nexus : They gonna heal you. ( It doesn't work against the other kind of turret )
_So : Galio is week in the very begining of the game, but youre control can chage the game, and you're one of the strongest suport in mid and late. Pick galio is a real risk, but If you reach the midgame without feeding, and with on or two assist, you will be a monster, in Late Game.