This is my first build and as with everyone it is just a trial at this point. Also make sure you use your head. If the other team has more melee dps then get GA before Force. If the other team is heavy on melee people then I sub out banishes veil for sunfire cape or thornmail. Both works wonders with your ult.
Galio is a very good tank and can help is team out very well. He doesn't have a global shield like Shen does. But his shield works just as well in team fights. And when casted on himself it makes him even more deadly especially with his ult. With bulwark on during his ult you will get healed very nicely and do even more dmg with the extra magic resist.
I like flash and exhaust for my skills. Flash so you can do your best fiddle impression with your ult and for even better escaping and chasing potential. Exhaust for the blind and to let your team catch up to them.
I updated the mysteries to utility. You really don't lose any survivability seeing as you can throw Bulwark up more often.
Also with the change in mastery you don't need the mana regen seals. I subbed them for flat armor and they work wonders.
Changed skill sequence slightly to do more burst dmg faster.
More updates to come. I am still experimenting with him. Feel free to leave comments/concerns about/with my guide.