First of all, I want to just let you know that I'm not that great at LoL, and its rare that I do well in games, so when I just improvised this build and found that I was just owning I felt I had to share it with everyone, so I hope you enjoy.

Whilst these may not be the best runes to use, personally I feel that its down to what you prefer to use. These runes are just the runes that I always use for most champs.

Greater Mark of Desolation x9, Due to the damage you do, it is most likely that the enemy will build armour, however even if they don't it will reduce their base armour, resulting in you doing more damage. I also think that these are good on any AD champ.

Greater Seal of Vigor x9, Good for early game Mana regen.

Greater Glyph of Magic Resist x9, I usually take these because it makes it easier to take on AP casters.

Greater Quintessence of Critical Chance More crit chance.

Greater Quintessence of Attack Speed More attack speed.

Greater Quintessence of Movement Speed A slow GP is a dead GP.

As I mentioned before you don't have to take these runes, these are just the ones I use you should at least decide what is best for your play style.

Again these are down to preference, but ill go through why I take these ones.

Summoner's Wrath : This makes Exhaust that little bit better, or if you want to use Ignite, its still a good choice.

Brute Force : More damage, More pain!

Alacrity : Faster damage, useful mastery.

Weapon Expertise : Good against anyone who has built armour, and even if they don't have armour, you'll still hurt them.

Havoc : Yet again, more damage.

Deadliness : Crits are always nice to have.

Lethality : More damage on crit, why not?

Vampirism : More survivability.

Sunder : Armour Pen. is always useful.

Executioner : Hell, why not?

Summoner's Insight : Reduces cooldown on Flash, Extremely useful

Expanded Mind : Goes well with Manamune.

Swiftness [2/4]: GP is always good with more speed.

You can change them how you like, these are just based on doing more damage and making the summoner skills better too.

Starting Items

Allows for a quick change into Tear of the Goddess aswell as early Mana regen.

Two of these to deal with wounds from early game battle.

Moving on

More mana regen, key component of Manamune and allows for build of mana before you can finish Manamune.

Boots, allows for the creation of better boots

Now here is the situational bit, from the normal boots you'll need to decide what is the best choice to go with
I usually go with one of two boots:

More attack speed

GP's 'W' doesn't save him from every stun/snare/taunt/slow and this item also gives some Magic Resist, which is always nice.

The Beginning of the End (The Core Item's)

Mana regen, bonus mana, more damage.

Build this over time, good when you combine your 'E' then use your 'Q', more mana(more damage), more health(more damage), crit chance, ability power(more heal on 'W', more damage on ult.), movement speed, more attack speed, more damage, awesome item and is fundamental in this build.

Armour, Crit chance, more damage.

Health regen to allies, Attack speed to allies, reduced armour to enemies, Lifesteal to allies.

The Optional item
After getting boots and the 4 core items, the last item is purely situational and up to you please bear in mind at this point, you should never have to go back to base after the last item, your 'W' can keep you alive. Here's some nice item's to use that synergise well with the core items:

Does magic damage to multiple enemies, plus more health(more damage).

Magic Resist is really useful at this point, good item to get as the blocking of one ability can save a use of your 'W', more health and more mana means more damage.

More mana more health(more damage), more ability power(stronger heal and ult), though this is just theory I haven't got this item on GP before.

Good for dealing with fed Tryndamere or any AD champ.

Makes you more tanky, and gives more damage.

More crit, more damage, harder crit's.

As I said before the last item is entirely up to you, but also its good for anything situational.

What Skills do I level first?
It is purely situational, if you aren't taking too much damage, max your Q first. If you are taking alot of damage, max W first. I usually don't max E first, but its up to you if you want to max it first.

There's an enemy! How do I kill them?
By the time you have your Triforce you should use your skills as follows:

Step 1. Use E

Step 2. Use Q

Why this order?
Using E will increase your damage by a small amount and proc Triforce, when you use your Q the Passive from Triforce and the bonus damage from your E will cause your Q to hurt, and if you get more crit chance, that Q will hurt alot more.

Your W should be used when you are stunned in battle, or if you are just very low on health.

Your R should definitely not be used to farm minions. Use it when you can secure a kill or if you are in a teamfight.

Do I really need to say why you should take this move?

It's what I take and I find it useful with dealing with AD champs.

I wouldn't really go for this but its up to you, if you want more damage and AP then go for it.

Other choices:

I don't tend to go for other summoners on GP, but if you don't have Flash because you're under level 12, Ghost is an okay replacement. If you want to use a different Summoner is fine, but change the Masteries accordingly.

Pros

Lots of Damage.
Lots of Mana.
Lots of Health.
Some Ability Power.
Stark's and 'E' help your team.
Some armour from Atma's.
Last Optional item allows you to deal with the situation.
You Are A Pirate.
'W' becomes a pro heal with the AP you have at the end.

Cons
Champs like Trynd and Yi will tear you apart if you don't play smart.
Kinda squishy.
Early game you wont do much damage till you get Triforce.

I Hope you find this guide useful, and if you have any feedback or suggestions you can leave them in the comments below.

Edit: Added more Cons.