For runes I take as much lethality as I can so I can farm up and become a strong laner to the mid to late game. You can also look into taking MR glyphs if you are going into an AP matchup, but be sure to cap CD with MAW, Ionians, Death dance, ETC
Down the cunning tree, I take TLD so I can pickup easy kills with Ult if someone manages to get away. Intelligence is also a good option over precision, its just preference. Bandit is an amazing mastery on GP as you can just Q your lane opponent and get 10 gold easy while at the process of poking them. Bandit is also a good mastery on him because GP is a punching bag early so you can make +1 gold every time you miss a minion.
Flash + Exhaust is a good safe summoner for a dueling lane like Zed, Yasuo, ETC. You can also swap out exhaust for TP if you feel your gonna impact the game from around the map or go back to lane without losing much.
Pros / Cons
Con to GP is that he has a BIG learning curb, learning barrel mechanics and getting used to them can take awhile to learn.
Pro to GP is that once you learn most of his mechanics and know your limits, you can easily become one of the strongest carries to impact the game.
Kill minions with Q ONLY (does not work if you kill a minion with a barrel with an auto) try to get AS MUCH silver serpents so you can upgrade to deaths daughter, the upgrade early is very necessary. I say have the upgrade by around 15 - 22 mins.