Gargoyle FTW amirite?! But anyways. Galio's one of my favorite tanks and I don't understand why he's not played more, except for the fact that he's the most expensive tank.

Pros:
Counters any AP champion due to his passive
Flash ulti is one of the best initiations in the game
Bulwark makes you tanky in lane even at level 2
Easy farming
Saves allies well, with slow/shield/whirlwind

Cons:
Ulti fairly long cooldown
Ulti can be stopped, unlike Amumu
Not as powerful in team fights without ulti
Passive useless vs AD heavy team
Costs 6300 IP

Use tank runes! That's about it. You can substitute armor reds for magpen reds, or I used to use mana regen yellow so I can sell chalice later on and still have good mana.

Spell penetration is nice, gives you a decent amount of DPS.
The defensive masteries are good cuz you're a tank.
You don't really need the exp masteries as much cuz you can still be tanky while being slightly lower level.

Flash is the most important summoner spell! Get in and ulti and gg.
I prefer Clairvoyance because I like to find the jungler and keep checks, as well as prevent ganks and seeing baron/dragon, but I only take it if my team does not have a support who holds it already.
You can also take Fortifyif you like.
Ghost can be good if you wanna catch people/escape if needed.
Exhaust gives you more CC!
Ignite if you wanna get early kills, but try not to KS your carries...
Teleport get back in lane=more farm or get into team fights faster, a must have if you take solo top.

Clarity is not necessary because you'll have chalice!
Heal ... I guess you can take it but I don't see why you would.
Cleanse If you think you need it, get quicksilver instead, more MR and gives you AP
Rally Lolololol
Revive ^

Resolute Smite gives you the most damage for easiest farming.
Bulwark gives you a lot of tankiness, as well as protecting your allies.
Only 1 Righteous Gust is necessary for the speed boost.
Level Idol of Durand every time possible.

Chalice of Harmony gives you the mana to stay in lane forever, and the MR gives you AP
Mercury Treads as a tank, you rarely get any other boots, plus the MR gives you AP
Sight Ward win games! Buy wards! Especially since you're a tank, one that can get high CS.
Heart of Gold just for the little bit of tankiness and gold per 5, and maybe build into a Randuin's Omen later.
Sunfire Aegis Gives you more damage for creep and to champs when they're pulled into you! Plus decent health and armor. I personally don't use it, but it's a decent item for him and I know a lot of people out there still like it.
Banshee's Veil bubble is always nice, it might prevent you from getting interrupted on your ultimate as well as give you more AP.
More Sight Ward!!! and Vision Ward on dragon/baron

Because you are a tank, items are always situational.

Firstly, Aegis of the Legion is always a good item unless someone else is taking it. Since you'll be at the center of team fights, your team gets nice all around stat bonuses, and it's a really cheap item.

For AP heavy teams:
Force of Nature gives you a lot of MR and AP and the health regeneration passive and move speed help so much.
Abyssal Mask gives you a lot of AP for a tank. It's really helpful if your team has a lot of AP as well, and is useful for all team fights since you'll be the center of all the fighting because of your ultimate.
Quicksilver Sash is especially helpful for those annoying stuns, like if the other team has an Amumu, or for those long surpresses from Malzahar or Warwick, but they'd be fools to waste surpresses on the tank.
Sight Ward

For AD heavy teams:
Randuin's Omen is an amazing item. It may lower your damage output on your ultimate, but who cares? You're the tank, you don't need to do the DPS. With this item you can lower the DPS of all their AD carries, as well as slow them down a **** ton. Not to mention the HP, HP Regen and Armor you get from this item is really nice.
Thornmail is decent, it returns all that damage when you ult them. And its cheap as well. I personally don't like it unless their team is almost all AD.
Frozen Heart is nice as well. Since you'll be at the center of fights, you'll lower all their attack speed. The CDR is really nice for your ultimate as well. And again, you're the tank. Who cares if your ulti DPS goes down a bit? Keep your carries alive.
Sight Ward

If you don't know what else to build, you might as well get a Guardian Angel Nothing wrong with being able to suicide while saving your team and then coming back to life to do it again :D. I rarely ever get it, but that's just a personal preference.

You can also get a Zonya's Hourglass If you need some armor and want that stasis. Plus all that AP so you can add more hurt to your skills.

Did I mention buy Sight Ward

Getting creep is easy. If you can get a solo lane, you can farm so much creep. There are many games when I get as much as, if not more than, our carries until I decide to let them take those huge minion waves. All you need to do is wait for the minion HP to drop a bit, then eye blast them. If you hit 3 of them you're bound to at least get 1 or 2 even if you're new to Galio. But don't forget to go auto attack them when they're low, or else you'll be too reliant on using Resolute Smite and run out of mana early game. Righteous Gust isn't really used for farming because of the mana cost and low damage. Unless, of course, you need just a tiny bit more DPS to clear the creep. But if you have a surplus of mana, go for it, do what you gotta do to get higher CS. And if the opponents think they can hit you while you auto attack, just throw up a Bulwark and you'll take little to no damage, unless you're being stupid. Remember to Clairvoyance when you're overextending, just because you're tanky does not mean you'll survive ganks if you overextend.

As much as I LOVE Galio, he's not the best in ranked play. In ranked games teams usually use an AP carry, AD carry, bruiser/offtank, tank and support, with a jungler somewhere in there. Most likely, they'll only have 1 or 2 AP champions, not allowing you to utilize your passive. The only time I would take Galio is if I'm one of the last picks, and the opponents have already picked mostly AP champions. If they're not AP heavy, I would recommend just picking another tank, as there are many more capable tanks for non AP based team compositions.

Flash Ulti! Flash Idol of Durand 'nuff said. That's what makes Galio so scary. But other than that, throw your Bulwark on yourself prior to ulting in team fights, or throw it on whoever's getting focused. Throwing it on your melee champions is also helpful since they'll be up close and personal with the other team, making them an easier target than your ranged carries. Be cautious of these champions when ulting:
Kennen will stun you if he has Slicing Maelstrom on when you ulti him.
Xin Zhao will knock you up if he has his Three Talon Strike on
Blitzcrank will knock you up with his Power Fist.
Udyr will stun if he's in Blazing Stampede
Leona will stun you if she has Shield of Daybreak on
Other than that, just try to catch everyone before they can stop you.

Galio is fun. You should at least try him when he's free :D But as for tanking, there are many amazing tanks, none of which cost 6300... You should get them unless you really like Galio.