Things to keep in mind:
GP is an extremely slow jungle, extremely mana hungry and overall there are much better choices. However, if you're like me, you find him extremely fun and are willing to take the slow start that powerballs into an extremely powerful squishy annihilator.
I roll with 3xLifesteal quints, armor pen reds or AD reds depending on enemy team, armor yellows and scaling magic resist glyphs. I prefer scaling magic resist for late game viability, and I only pick GP jungle when my mid won't need blue. (Kennen, for example) The lifesteal quints I feel are very important, it provides just enough sustain in the jungle to successfully clear. My mastery page is standard AD/tank. My starting items are usually cloth armor and 5 health pots so I can safely gank and clear without having to B, however, you can go boots/3 health pots if you want to gank after blue and then go back and buy, or you can go with a vamp scepter if you don't plan on ganking much or your lanes are going to pushed. Cloth armor/5 pots is usually a much safer choice. I get my E first, then Q, then W, then rank up Q.
I start blue (don't use smite) -> wolves -> red (use smite) -> gank (usually top, sometimes mid if over extended; very rarely bottom unless I know they don't have wards)
GP ganks are absolutely lethal, you land a parrrley and you're either going to force a summoner or get a kill. When you come out to gank, whenever possible save your E for when your lane champion is in range so you both get the benefit; this will make a huge difference.
The first item I rush is a Madred's, then boots, then philo stone, then wriggle's. In between I will usually buy a ward for top lane (river bush or tribush) and enemy wraiths. Knowing the enemy jungler's location lets you safely counter jungle, safely gank, or counter gank; it can really secure a win.
Beyond philo and wriggles items are situational, but you should be out of laning phase by this point. If you can safely dragon squeeze those in, don't be afraid to use your ult to save a tower and keep wards out!