After reading over the abilities at hand, i can tell that the slow and speed increase will make gal a very strong ganker. i start with masteries in all the major offense and focus on galio's primary tank status. I would rush chalice, since the magic resistance and mana regen are never bad and give me bonus AP. Banshees appears to be one of the best things for gal as it helps him get out of that one spell that is a difference between a kill and a failed gank. Here is where there is some disgression, i would rush lichbane for the early kills but honestly the abyssal will be one of his best items. once those are done we can get to end game and tank. GA and force of nature should be more than enough to handle their nukers, but we will see.

Pros:
Magic resistance adds to ap making abyssal lichbane the way to go
banshees will get you out of harms way as well as cleanse
The GA is just to be annoying

Cons:
Takes a while to farm and it will be critical to make the right choices on items.
Not as much HP as one would like, but thats why we have guardian banshees
lastly i don't have top tier AP i just have enough to be a threat in team battles.

GLHF Hope this works for everyone