In this guide I will show you my personal favorite way to build gp and maximize the damage out put to the MAX!(kinda :P)

Marks
For runes i like marks of furor for the extra critical damage along with brawlers gloves if u get a critical at level 1 you are going to be hitting for about 250 - 350.

Seals
I take seals of alcacrity just for the early game attack speed nothing major but enough to keep me from thinking "I attack to slow i need to change item builds"(don't judge me).

Glyphs
Glyphs of replenishment ftw.The glyphs of replenishment are very helpful for me because I tend to spam my q to a point where i have no more mana for a big fight,so i started using glphs of replenishment and the mana regen really helps.

Quints.
The last one,Quints i take armor pen quints aka Quints of desolation(who doesn't get armor pen on ad carries O.o?).

I don't really have anything to say on items just follow the item build and you should be set.

For skills I max out my q first then w,r,and e
I never really liked the e and still don't and it doesn't really seem all that important to me early game(late game's a different story though).
Gp's w is the anti cc it breaks you out of any cc(which tends to piss off a lot of malz and ww ;).)and it heals you once activated.
Q is maxed out first because by level nine it crits for about 500 no items needed with items like bloodthirster and I.E you will be criting over 1,000 easy.
Last is the not so great ult of gp which to me is for finishing someone who escaped you or initiating a team fight.

FLASH for escape and chasing IGNITE for well igniting them to reduce healing or letting it kill them early game :P

Pros:
great burst damage(late game)

Q can farm very well early on

Can escape from any and all cc with w

Cons:
very squishy with out health increasing items(but i fixed that issue with trinity force ;))

Can't chase very well(HEEEEY I fixed that too rite? sorta)

no cc of his own(yes I know his passive slows but its not very noticeable so it doesn't count)

When starting a team fight always ult because it can change if you lose or win the battle

If a teammate is about to die being chased by enemies try your best to help if you are close enough jump in and rape with all your crit power if it gets to be to much flash out or if you are like cross map away ult to try and slow the enemy enough to save them(its on a pretty low cd so its ok)

If it turns out my build helped you please comment saying so if it didn't well comment say so,if you think there are ways i can improve my build i'm open to suggestions.