i dont have much to say about the guide, i think its kinda standard but that might be because i've always built gp the same, even before the patch.

armor pen marks just increase you damage, especially against tanks (especially amumu).

the att speed seals for more crit hits, which keeps you like longer and kills them faster lol

the cooldown glyphs so you can QQ more, which is a huge part of GP i think

and the quints for MOAR DAMAGE

standard AD masteries, although if you prefer the extended buffs over the gold thats your choice, i prefer the gold since his passive is basically a scaled-down lizard buff

ALWAYS start with brawlers, you need the crit to force out your opponents early on, with help of your Q (hopefully critting that). afterwards i build it to an exectioners calling for the lifesteal, with some increased crit. i like the sustainability, because if you can force them out of exp range with Q, you can farm minions and HP

after that beserkers for some att speed and movement. this helps, along with your passive, to ensure more kills

i got for the zeal next for all 3 of the bonuses. if your criting enough already you may want to get a BF sword first for the guaranteed damage, which in turn raises crit damage. the rest of the items just build on the core ones. you may have noticed that there is an extra PD in there. i sell serkers to buy it because your E gives you enough movement speed and the PD just increases crit more. you can never have enough crit.

some GP's like E more for the ad and movement speed, but i prefer to harass and spam Q which is why i level it up more. i dont put much in W because with the lifesteal early on you really only need it to get rid of CC

i bring either flash or ghost, and ignite. you could bring exhaust as well if you like it, it definitely helps if you need the damage reduction, or if you want to troll this so they more so slow that you can auto-attack them to death.

grab blue and red buff whenever you can, red for kills, and blue for more spamming power, because he is a mana-***** if you spam E too much (like me lol)

early game get those last hits, late game you should be 1 shotting them so just hit them. also if you think you can do without your ult for awhile, use it on a tower that has snowballed enemy minions around it, like those OP purple caster minions.

MAP AWARENESS IS KEY!!! watch the map for fights and you use your ult to:
1. slow enemies to save teammates
2.slow enemies for allies to chase
3. last hit champs that got away