This is my build on Gangplank. I play him as a DPS, like to dodge things, and rely on stealing lots of life. Roll in and crit some people down baby. This is my first guide so bear with me and lets see if this makes sense. (Don't knock the name of the build either, I'm not really that cocky, I just really like Pirates of the Caribbean)

7-11-11 Got rid of Youmuu's Ghostblade in favor of Phantom Dancer I like the attack speed coupled with the higher crit chance and more move speed makes you a very mobile fish out of water. More to come soon.
7-12-11 Added reasoning and numbers on attack speed and crit strike behind switch to Phantom Dancer. Also got rid of Ignite mastery, really no need for that in this build and I feel silly having it before.

Pros:

Can stay in lane longer early game
BIG Crits middle game
Extremely Strong late game
Better survivability with life steal
Ultimate range on Ulti

Cons:

Squishy early game
EXPENSIVE (have to be good at farming)
Can be focused down (but almost anyone can)/gets targeted early
Tough to make up a rough start

I go with Armor Penetration Mark (makes sense with Parrrley), Dodge seals (works well with Ninja Tabi and masteries), CDR per level glyphs (it works nicely with your cleanse, plus it never hurts to get Parrrley or your ulti off one extra time), and Armor Pen quints (to make your attacks hurt that much more).

Side note on glyphs: A lot of people go with just flat CDR here. In my mind the reason you go with flat Armor, Magic resist, Armor Pen, etc. is that you're using them mostly early game. Once you get toward the middle and late game, your builds take over in those statistics. CDR is something that is always helpful and works as a percentage as opposed to a flat number and is something that the other players can not build against. So the more you can get of it, the better. Someone can build a Thornmail if you stack AD, but no one can do build something that'll effect your cooldowns (thats an accurate statement yes?). Long side note but I thought it warranted discussion.

Very offensive here (I know that you could make a joke with that if you don't like these choices but I'd like to think you could be a little more creative) going with Exhaust and Ignite. That combo has helped me out in numerous situations, plus with the mastery in ignite you can farm just a little bit better when it's on cooldown.

Your choice though as to what you want to go with, I would say always have ignite on you. Other viable options:
Flash: always good for escaping and positioning in a team fight
Teleport: Useful to have if no one else does, can get you to a turret to clear minions, can also get you back in lane quickly if you need to.
Ghost: Good for chasing or escaping, fun to use it and remember it being one of your only options at level 1.

That's about all I'd consider using. It's your call

I go 21-9-0 on the masteries. Dodge, dodge, dodge, it's very useful in almost every game, plus is fantastic if you have to go toe to toe with someone like Tryndamere or Xin. It's also nice in early game so minions don't pick away at your health. It's your choice if you want to go magic resist or armor on defense, I personally like MR just because you're going heavy on dodge with this build and will hopefully skip around some of those auto-attacks that are doing physical damage. I also like to max out ignite and exhaust, but those are flexible depending on what you choose for your summoner spells.

If you don't want to go all offense with your summoner spells you can go 21-8-1 and grab a point for ghost.

If you don't want to go dodge then you can go 21-0-9 and grab a different set of boots, but again that's all based on your playstyle.

I max Parrrley first because it's great for harassing, is going to be your biggest punch the whole game, and... FARMING! It's a pretty expensive build and you are going to have to farm the **** out of the game to get where you want to be. Plus the extra gold you get on kill shots really add up as the game goes on. Next you want to max out Remove Scurvy, grabbing a point in Raise Morale at level 4 to help in team fights. Remove Scurvy is going to keep you in lane longer (healing and cleanse, too good) and let you keep farming until you have enough to grab Sheen and possibly a Boots of Speed.

A lot of people like to grab one point in Remove Scurvy and max out Morale first, I like the ability stay in lane longer with the healing that you get with the increased levels there.

I start with a Sapphire Crystal for the mana/builds into Sheen and grab two health pots to stay in lane as long as possible. If you're doing well the first time you go back you'll be able to finish Sheen and get at least some Boots. Next time you go back, finish the Ninja Tabi and grab a Vampiric Scepter. That finishes your core build: Sheen, Ninja Tabi, and a Vampiric Scepter. (If you want to hear the explanation on Sheen for GP read Aeneas' guide, he goes into some pretty convincing detail).

From there it depends on what is going on in the game. If things are going well you can finish off the Scepter into a Bloodthirster (to start getting those stacks early). Then grab your Cloak of Agility and finish off the Infinity Edge. From there you can decide if you need defense and survivability ( Banshee's Veil or Guardian Angel) or want to keep going offense ( Trinity Force). If you haven't done as well as planned it doesn't hurt to get a Cloak of Agility or an Avarice Blade (that depends on your gold needs, if you're hurting for money the gold per 5 can be helpful) and start getting some crit to help grab some kills.

Why I like these items:
Sheen: Read Aeneas, it just makes sense
Ninja Tabi: By getting the dodge seals and maxing out the dodge masteries this just makes sense. It's incredible how much this helps, especially against someone like Xin or Tyrndamere. I can't count on my fingers or toes the number of times I've watch dodge pop up above my character fighting those guys or even turret diving. It's a bit up to chance whether or not it works, but it's helped me enough that it's a chance I'm willing to take.
Bloodthirster: This is where I deviate from most GP builds. A lot of people go straight for an Infinity Edge here and I understand why, the crit it gives is incredible. And thats why I have it in the build. I like Bloodthirster beforehand though because it allows you to survive longer as well as jungle at will. By the time you get this item you technically could solo the dragon (maybe not advised yet, but I've done it).
Infinity Edge Huge crit chances on Parrrley. Just with a Cloak of Agility you can have crits of up to 700. Now add the rest of the item and you're sitting pretty.
Trinity Force You have the Sheen already, plus it's just a great overall item. Extra crit, extra attack speed and Phage works really nicely with your Grog Soaked Blade
Phantom Dancer I originally had Youmuu's Ghostblade in here but after trying Phantom Dancer wouldn't go back and here's why:
Because we're stealing life it makes sense to attack as often as possible to steal as much life as quickly as you can. You add Phantom Dancer with the Trinity Force you've just bought and you're doing 1.5 attacks every second. Lets consider this: If we attack for 10 seconds we're getting 15 attacks in. At 70% crit chance about 10 of those are going to be crits at 250%. We're going to get 2 Parrrley of in that time as well. Lets put the Parrrley crit at 900 damage (I've had crits of up to 1200 with this build so 900 is relatively safe) and doing 278*2.5=695 damage before armor so lets put it at an even 600 for crit strikes with your auto attack. If you do one out of two crits on Parrrley and 10 out of 15 crits on your auto attack that means 900+360+10(600)+5(278)=8650 damage in 10 seconds. That means that you just healed yourself for 8650*.15= 1297.5 hp. This is a low end estimate and is also before adding in the boost you get from Raise Morale.
Defensive Items:
Banshee's Veil or Guardian Angel depending on the situation. Getting focused down early in team fights? Get the Guardian Angel and be back in the fight stealing more life and laying the smack down. Got a lot of guys with nukes? Grab the Banshee's Veil and block one of them.

Be flexible though and adjust whenever necessary. If you are getting tons of magic damage consider a Hexdrinker. If you have the money to grab a 2nd Infinity Edge early and it fits the game, go for it. Remember, this is a guide, not a formula that works for every situation (although I've found it works for many).

So thats how I like to play GP. Props to Aeneas for the Sheen look. Please try it out before you vote and let me know what you think. Definitely open to constructive criticism. With that said, "Haters gonna hate" and I know that, so give me your honest opinion and we'll see what happens. Thanks for reading.