This is my gangplank build, i have used it for a long time, and rarely ever go negative on it. provided your not an idiot, this is a very good build.

Pros:
strong DPS
reduces healing on enemy champs
can heal him self
can remove 1 CC effect
high movement speed

Cons:
Squishy early and somewhat late game
low mana pool
no CC before level 6
targeted in team fights(but not always)

As for runes, I get armor penetration marks and quints so that early game parley does more damage, and later game you have some armor penetration before you get the black cleaver. Armor penetration is VERY good early game as many champions don't have much armor, and those that do, you will go thought more of it than other champions.

The reason I get critical damage seals is that when I do crit at any level it will do more, early game critting is amazing with this because you do not yet have the infinity edge. Until you get the infinity edge the extra crit damage will be amazing. With the armor penetration you will be doing even more damage.

For glyphs I take cool down reduction so that I can use parley more often, or in tight situations there is a lower cool down on remove scurvy so you can heal and get away, hence dieing less.

For mastery's i take a 21/0/9 set up.

The reason i do this is for more attack damage per level, and increased attack speed, crit damage and penetration.

For the utility mastery's, i take these so i can have a little bit more mana at the start of the game, and some more movement speed to have a bit of an advantage against other champions who don't have this utility or boots.

So to begin the game I start with brawlers gloves for the extra crit chance with parley so that you can farm or harass better. When you have enough money go back to base and turn that into an advarice blade. Also if you can buy the boots of speed, or go back later to get them.

Once you have the advarice blade and boots of speed, I get the zeal for increased atk speed, crit chance, and move speed (helps cover for not having full boots yet). After the zeal you should be better at farming due to higher crit and faster attack speed. Then get your berserkers greaves so you have the full boots + zeal for good speed to catch enemies that like to run (hopefully). This will also give decent atk speed.

Next get the cloak of agility, once again for the higher crit chance with parley for harassing and farming. Then get the B.F. Sword when you can afford it. This is the first real damage item you will have, the additional damage will stack nicely with the crit chance that you already have, and the crit damage from the runes. Finish off the infinity edge ASAP.

After the infinity edge turn your zeal into a phantom dancer so that you have more crit chance, more atk speed, and more move speed. This will make you run faster than more enemies, + raise moral will make you run even faster.

Grab another B.F. Sword for extra damage and a vampiric scepter when you can. This will turn into a Bloodthirster so that you have some life steal for higher survivability when in team fights, although its not a lot, it will work nicely with your remove scurvy.

Now, remember the armor penetration I mentioned earlier? yeah this is where you get yet ANOTHER B.F. Sword to make a black cleaver so that you have increased attack speed and armor penetration, and the black cleavers ability is amazing when fighting tanks. The armor penetration stacks perfectly so that you can take the tank down quickly and efficiently.

Lastly when you have enough money, which at this point should be easy because you have around 55% crit chance and a decent amount of damage, you should sell the advarice blade and get another bloodthirster. This will give you more attack damage, and more life steal giving you a bit more survivability. Don't worry about your crit chance though, it may be lower your crit chance, but grab the elixirs when ever possible. This will give you some more mana, crit chance, and more health. Although mana shouldn't be a problem at this point, you got nothing else to spend all that money on anyway.

Although you do not get 100% crit chance with this build, you still get a decent amount of crit chance, with a great amount of damage, especially with maxed out bloodthirsters, and a good amount of crit damage.

I start with parley because it is the main skill used to farm at early levels with gangplank. Also it is a good harassing skill, but I would just use it on minions for the extra gold. I try to max this skill out first.

At second level it really depends on what you are going against. If its someone who likes to harass I would recommend getting the remove scurvy so you can heal a bit of the damage that you take. I try to max this second.

At third level grab the skill that you didn't at level two. I tend to get Raise Moral at level 3 because I don't really use this skill at lower levels except when trying to get away because it uses a lot of mana at lower levels. I tend to max this last.

Upgrade your ultimate when ever possible.

I usually take ghost, and ignite. ghost is helpful at any point in the game. helps you get out of fights, or even catch up to people to get the finishing kill. yes ignite may seem useless because of your passive, BUT ignite does true damage, and scaling with your passive, is godly against characters like mundo, and warwick, and other people with life steal.

Also i find that taking clarity over ignite would work too. if you dont want ignite because of your passive, than clarity will help because it allows you to spam more skills early game.

Other useful spells:
Flash - get in and out of fights, ganking
Exhaust - This helps in slowing the enemy, and lowering their damage against you.

Other spells:
i havent really used any other spells on gangplank...
but those i know that will not help you are rally, revive, smite, fortify, and heal.
Mainly because you are not a jungler, not a support character, revive is useless on just about everyone, and you already have a heal.

Farming on gangplank.
Early game you are going to want to use parley on minions to get the extra gold it gives you. having a Twisted Fate on your team helps with this tremendously but parley gives you between 4-16 gold depending on the level. Also the cool down glyphs will help because parley already has an insane low cool down, making it that much more spam able.

Late game you are going to be able to wipe a creep wave in a matter of seconds, 1 hitting just about every single one of them you will be a very good pusher and farmer.

in team fights, try and use your ult where it would be most useful for your team, unless there is that one straggler running away that has a nice bounty on their head that your ult can kill. :P

what I find best for playing this build, is to go into the fight with the tanks, mainly because you will do huge damage, and using parley will bring the other team's squishy champions to low health before the fight has really even started, this also makes them scared of you, so when they see you, they might hesitate to fight, but they might not.

Like i said at the beginning of this build, this is a very useful setup to have for gangplank, does a lot of damage, and has decent survivability. Also like I said earlier, you only have around 50-60% crit chance so you will not be critting all of the time. On average once ever two swings. Even then your attack speed is very high and your attack damage is great. So you should not have to worry about doing crits at all. They will come and they will be high.