Yikes. Uhh yeah, he's pretty decent.
Personally what my team usually runs when I play GP: Gangplank top solo with smite and exhaust/cleanse/flash/Ignite. If you're playing draft mode the ladder becomes more obvious.
Onto runes usually you want to go Ad, armor pen, or even crit chance reds, Armor yellows, magic resist blues, and Hp, Ad, or Armor pen quints. So you can run about any standard physical runes on him.
As for skilling on GP You will want to take 21/9/0 masteries making sure to grab the smite buff. It doesn't seem like much but you can propel yourself into mid game quite quickly. Your First 3 levels are what's important. Parrrley at 1 Raise Morale at 2 and if you are doing very good take another rank into Parrrley. If you aren't doing so hot take a rank into remove scurvy at 3 for safety. From there you want to remember Parrrley>Raise Morale>Scurvy.
The reason you take smite, and top solo is, because Gp isn't exactly the best jungler until you get your hands on some items, but when you do you can trade off your lane and go into the top jungle for a while. So you start off by waiting in the top jungle on the camp closest to your base, smiting immediately and following up with parrrley for the last hit (+14 gold before your enemies have earned any!) finish the camp off, and head to lane.
Oh, and start with cloth armor 5 health potions.
Now you just do the usual, last hit with Parrrley, and harass when you can. You should probably back around level 4 or maybe even 5. At this point you probably have 2-3 hp pots left, and your smite is up. Go home and buy a madreds razor, a couple pots, and a ward. Rush to top jungle, and place that ward at dragon. If a team mate hasn't already taken a solo lane inform them, and then go to farm top jungle.
After that there are just a few things to take into consideration item wise. Boots can be almost anything: YER PLAYING GEPE FER GOSH SAKE. That madreds razor will typically turn into a wriggles, unless you see the enemies severely hp stacking win which case you might want to go blood razor. Early avarice into yomuu's is always nice, but replaced by a last whisper quite easily. From there just build general dps/bruiser items. Make it well rounded.