Yes I know there isn't much here compared to other builds, but wth.

Galio is a great counter to most Ap Mids. His passive means you can stack MR and still pack a punch. You are also so tanky that you have to play stupid, have really bad luck or get greedy for a gank to work. Harass with you Q and don't push the minion wave, it is REALLY easy to do this since your Q is the easiest way get cs. Try to keep your minions just outside tower range so you can melee individual minions easier. Don't start majorly harassing your opponent until you grab the Chalice, GO BACK THE SECOND YOU GET THE GOLD TO GUY IT. Depending on your preferred game play and your opponent, you may wish to get Maki instead of Null-Magic Mantle. Once you get the Chalice, rip the enemy mid a new one, force them out of lane with harassing and call in some ganks. At this point I would grab wards so you push to their tower. (Galio is terrible tower killer. So if you can, have the jungle with you when you push to the enemy tower)

I have Zhonya's Hourglass because it will allow you to survive a normally suicidal flash-ult in late game.

You could build that into a Deathfire Grasp, but I really don't think Deathfire is suited for Galio.

Item Alts for Force of Nature and Abyssal Scepter:

Rabadon's Deathcap - Great for any AP. If you feel that you are a bit under powered definitely throw out Force of Nature or Zhonya's. Get this right BEFORE OR IMMEDIATELY AFTER BANSHEE'S VEIL.

Spirit Visage - Survivability & CR

Morello's Evil Tome - AP & CR

You have two jobs. Protect and Harass. Your number one priority is your AD carry. Anytime you even think your AD is going to get hit you should cast Bulwark on him. If your AD needs to get out of there quick cast Gust in the right direction and lay down covering fire with Smite. (Which is also a slow) Your next most important task is to make the enemy AD's life a living hell. Your smite does great damage and slow, but the attack takes an eon and then some to reach its target so you should shoot it from the bushes. Watch for when the enemy AD goes for a last hit, shoot your Q where you think the AD will take the shot from. You need to land these and make sure the AD pays for every single minion kill.

Once you get your ult watch for chance flash in and ult for a kill or an assist. Baiting them to dive you under tower is one of the surest ways to get a kill as Galio. Don't be afraid to use your ult to save your carry if things get sticky.

In team fights you HAVE TO ULT THE AP AND/OR THE AD CARRY. Depending on how fed the carry is, it is acceptable to ult a carry alone. Its not ideal, but its better than letting the enemy carry rip your team to shreds. I would suggest you save Gust in team fights since it doesn't do much damage. You want to keep in reserve in case things go south and you need to GTHOOD, or if you need to chase down a champion.

Alternatives for Force of Nature, Aegis of Legion & Abyssal Scepter:

Force of Nature - Crazy good MR & survivability.

Guardian Angel - Straight survivability. Get if your getting owned.

Randuin's Omen - Kick *** item, great for any tank support.

Shurelya's Reverie - Good item to get since you've got the first part already, great for Galio if you need to help chase someone down or GTHOOD.

Spirit Visage - Cooldowns, MR, Health Regen. Balanced.

Yup, just thought I'd throw this one out there. Enjoy, and yes I know its short, so sue me.