Ok here goes, Gangplank, many people seem to think that he doesnt do that much apart from his critting gun and his slowing ulti but nooo, he can dps any champ to death because of his deadly crits. You use parrley, crit them and they get reduced to half hp and then u go in and start attacking them. 3 or 4 hits and they r dead. Im going to tell you how to achieve this.

Gangplank is a very good overall champion against tanks, dps champs and hybrids - however he cannot stand burst-damage champs and nukers such as Ryze, Akali, Karthus, Leblanc, Lux...

PROS:
-Is kinda of a good nuker since his gun crits for lots of damage
-Has an effective self-heal (that can heal even when stunned)
-His ultimate is very good in team fights and taking down an enemy from afar
-He can deny quite well if you have alot of mana regeneration
-Easy use of abilities
-Decent farmer
-Is excellent at 1v1-ing and against any champion except burst/nuker champions

CONS
-Is squishy early game (until you have your heal)
-Can easily be CCed (crowd controlled)
-Susceptible to nuke and burst damage
-His parrley ability is based on the luck of a critical early game so don't rush into tower to use parrley thinking you'll definately get the kills, cos u might not.

I take ignite and ghost for reasons listed below:

Ghost is very useful not only to get away with Gangplank (don't have to use if you have your 'remove scurvy' ability) but to chase low health enemies into towers and get out alive. Gangplank is very effective at jumping towers because you are one click away from shooting a bullet that could give you a kill, or to land your ignite. Ghost is also useful when rushing across the map to intercept an enemy fleeing on low health

Ignite is extraordinary because at the start of a 1v1 battle or in a team fight, you can use it on a champion you think is good at fleeing or the person you are attacking. So you have a better chance of getting the kill if they attempt to flee because you're too awesome (and in this case, you are XD)

With runes, I take the standard Critical Chance runes so you can have 'lucky' criticals early game with either your basic attack or your parrley. It is very effective at scaring your opponent if you land a hard crit in the very 5mins of the game. They will then ask if you have critical chance runes and you say 'yes'. It is a good scare tactic. I also take a bit of attack speed because mid-late game, Gangplank likes to beat the **** out of any champion and as a pirate, he loves to pound them into lard. I With the glyph runes, I either take cooldown reduction runes so I can have quicker access to spam my parrley or to heal myself OR take mana regeneration so I can deny early game easily without losing lots of mana. Or to prevent being oom in a solo lane. Additionally, I take critical damage because it just increases the damage dealt from the criticals, its just nice to have.

My skill sequence is a bit straight-forward. The main idea of getting 'Raise Morale' first is to deny the first creep to the enemy champion. Then as you progress, you deny every tank minion which comes to further deny some..not alot of experience from your opponent. I then try to max out parrley because it is such a good ability to spam on enemies as they would get very annoyed. When i reach Level 4, I take 'Remove Scurvy' because it is good to have a heal early, so that you can stay in-lane for much longer. After that, i take my ultimate of course, max out parrley and the heal and just finish off with Raise Morale.

In my build, I like to take Brawlers Gloves because of the critical chance, then a health pot and a mana pot. After that I buy my first avarice blade to further the chance of a critical hit, finish off my shoes and purchase another avarice blade.
My Main Items are:Ninja Tabi because it provides armour and dodge chance, with Gangplank that is not needed but it helps.I take Phantom Dancers early because with the additional critical chance from the runes and avarice blades, I can start getting some serious pawnage happening.I Take Infinity Edge for the attack damage, the further in critical damage and a huge amount of critical chance.Another Phantom Dancers for the movement speed and critical chance. The movement speed really helps you chase and run away, it is also useful on getting around the map with ease.I get a Sword of the Divine mainly because Gangplank has lots and lots of attack speed mid-late game and this helps dish out additional damage as it deals 100 extra magic damage on every fourth hit. Really fantastic!My final item is Bloodthirster for some health lifesteal just to remain longer on the map and not have to recall. Any better suggestions for a last item is welcomed ^^

EARLY GAME:

During this time, you dont want to attack them yet. Just keep creeping and staying back - letting them think you are easy to kill. When you first start the game, stand near the nexus and wait for the minions. When minions spawn, shoot the first minion and walk with them. When u reach the end of bot or top lane, shoot another. If going mid lane, you will only get the chance to shoot one. This will reduce the enemy's experience. While laning, you get to lvl 2 and get parrley. Don�¢ï¿½ï¿½t use it alot, use it occasionally just to scare them with the hopeful crit, to cause them to back away a little. When u reach level 4, you get ur remove scurvy and you can start healing urself if u r taking damage. At this time, you can creep all u want and if they come close, just scare them away with parrley. Now when u reach level 6, tell ur team to shout out when they need the ulti or use it to gain a kill. You should have ur avarice blades and should be able to have a good chance critting. Slowly deplete ur enemies hp by constantly finding the chance to shoot them. When they r lower than half or at half hp, use parrley, activate ur ulti on them and go into melee fighting with them. You should be able to crit them while meleeing them and if they run away at like 20hp, DO NOT FRET - they will die by grog-soaked blade haha.

MID GAME:

During midgame, you should be going to bot or top for occassional ganking. Using ur ulti for when they try to escape not to initiate. To initiate, use ghost - fire parrley and start auto-attacking them. Every 3 or 4 seconds, parrley them again. When on low hp, use remove scurvy. Do not be afraid to retreat a little if things get bad, because they r sure to follow u. Once remove scurvy is up again, regain ur hp, and start annoying them with parrley again and this will guarantee you a kill. Remember to use raise morale to basically up ur ante in attack speed. Although your parrley is the main damage output for gangplank, always go into a teamfight and start auto-attacking them. Because with phantom dancers and infinity edge on the way, you should do aroudn 400 crit damage almost every hit. Now this is only because of the runes, my runes give me 100% crit chance when im in mid game and the crit damage runes give me more damage for every parrley obviously. But you have to be more careful early game, if u do not have the runes to give u a deadly edge early to mid game. Start defending more until you have around 35 - 60% crit chance during midgame. When you reach the end of the build, you find that u cant fit sword of the divine or bloodthirster. This is where you substitute one avarice blade (sell it) and afford the sword of the divine. Then when u have that and the game is still going, you should sell the second avarice blade to buy a blood thirster. Note that ur crit chance will then only be 96% but its still a very high chance of critting every auto-attack and parrley.

LATE GAME:
The best thing i love about gangplank when its late game is how he is able to crit squishy characters like ashe/fiddle/corki/evelynn/shaco/yi in barely a few hits. Parrley should deal 1/3 or 1/2 of their health and then u go in and auto-attack them or [crit] them to death. If u feel that ur gonna die by a pure dps character like yi, rmemeber to use remove scurvy. And then parrley them again. You should be using ur ulti for every teamfight and to help others either escape or counter-gank. Dont hesitate to help someone out using parrley and auto-attacking them to distract them. If u have a warwick/shaco/yi or olaf on ur team. You and [any of the characters ive mentioned with lifesteal/dps] can do baron without the other 3 on your team. Characters like shaco with his jack in the box is useful because gangplank can raise morale and use it on the box. Effectively doubling the attack speed and getting baron in less than 5 seconds.

Remember to not be afraid to go into a melee fight with other champions using auto-attack to deal bonus damage, parrley to crit them and effectively destory half of their hp and using remove scurvy to heal urself.

Remember to always use parrley to harass, do not go in and intitiate, just fire the occassional crit into the midst of enemies.

Be conscious of your health and remember to use remove scurvy. You can use it even if ur stunned for some reason. Like veigar's stun or udyr's or even amumu's. Quickly tap the keyboard key 'W' and heal urself - perhaps even saving ur life and maybe gaining u a window of opportunity to kill someone.

Always look out for enemies who are low health or allie who are in grave danger of dying. Be aware of what is happening around the map as well as taking 'care' of your team mates.

Hope my guide helps. This build and guide should effectively boost ur skill in playing gangplank because tanks and dps characters do not apply to gangplank since he crits everyone 100%, no exceptions ^^.

Here are my Scores, i will upload screenshots soon:

22/0/11
24/8/16 [2v5 game where i effectively crit kassadin/poppy/ashe to death in a matter of moments and shaco who was with me, killed the other two.]
23/2/9
17/5/6
13/5/16
9/2/6