Gangplank is considered by many to be an overpowered champion. I think he is an annoying but balanced champion. Gangplank can clear jungle extremely fast and can support his team from anywhere with his ult. Having good map awareness and being ready to use your ult to help your team is what will make you a good GP player.
There are many ways to build GP and there is not really one right way to build him. This however is the build I prefer.
Mid/late game I do a ton of split pushing and try to get as much farm as possible. FARMING IS EVERYTHING so make sure you go for farming/pushing over kills any day.
Replay for GP
I rune for fast jungling. I balance armor pen with attack speed so that I can hit 6 fast. The attack speed helps you stack your passive on things much faster. Plus I get last whisper early so the armor pen is not that vital.
I use Mr per lvl blues and armor yellows which are standard melee dps runes.
I use 21/0/9 standard physical dps masteries.
Gangplanks core is Wriggles+trinity+last whisper. After these three items you build will depend on what your team needs. If your team needs more dps build Infinity edge+banshee. If your team needs more tank then grab warmogs+atmas.
If the game goes super super late game you can sell your boots and buy a phantom dancer in its place.
I start off with E and max this first. I grab one point in Q+W at 2 and 4 then leave them until later levels.
I use flash smite. Flash is crucial for escaping as well as catching up to other people who flash.
http://www.youtube.com/watch?v=vhFhIk1v_HQ I follow this exact path for jungling gangplank
Video credit goes to stonewall008
This path is good because it gets you to 6 the fastest. You can also do armor+5 pot and start blue buff with a pull but I dislike this path.
Mind Set of Gangplank
Gangplank in team fights should mainly be using parley and minimal auto attacks. Only go in to auto attack if cc has been blown or you are chasing someone down/cleaning up. It is very important that you understand positioning to be a good gangplank.
Also don't forget to use gangplanks W to remove hard cc!
To gang with GP simple use e then run up to someone and slow them with parley. Then you can either drop ult slightly in front of them or just go for the auto attack spam. Ganking with gangplank is very simple and straight forward.
One thing I like to do with Gangplank is have bottom lane push up all the creeps to the enemy tower. Then I wait in a one of the bushes for someone to walk into it. I E then mow the person down. Any squishy that face checks will die.