Focus on farming with Q+E early game until you get your chalice, which should be easy as it's only 890g. It's an insanely good item on Gailo, giving you 30MR, unlimited spamming potential with no mana problems, and 15AP due to his passive. It's like a doran's ring but better in every way for him. If Galio can keep a farm advantage, he'll do incredible damage, especially if laning against a squishy ap champion. I usually build chalice, merc treads,aegis (actually a pretty big damage boost), rylai's, zhonya's, banshee's/perhaps fon, and by then the game is usually over but if not just max out your mr and defensive stats as your damage potential will have gone down by a lot. Sell your chalice and aegis for more powerful items late game.