Well, it's my first build, because i just wanted to post a build that I always make..
I want to thank you for coming to see my build, i don't know if you'll like it, because i think you'll not, but anyways, thank you!
Hope you get to play well with GangPlank with this build.. (:

First of all, you should ALWAYS use Parrrley, no matter if the enemy is with full HP. Because you can hit him with a critical and take his HP out... When you get Slowed/Stunned/Snared/Taunted/Feared/Silenced you should use "Remove Scurvy", so you can remove those effects and get some HP back... When your team is going to destroy a tower or to a team fight, you should use "Raise Morale", because you give Attack Damage and Movement Speed to yourself and allies... Your ultimate is super important, because you can use anywhere, principally when allies champions are chasing someone, you can slow them with your ultimate.. You can "Kill Steal" with your ultimate, or even kill those who are running away with very low HP..

When you're playing against him, you should be aware of him coming, because he'll try to hit a Critical on you.. You should also be aware of his ultimates, and also be aware when he uses "Raise Morale" because another enemies may be coming, or they're trying to "Gank" you.
You should NEVER try to Exhaust/Stun/Slow/Silence/Snare/Taunt/Fear him, because as I mentioned before, he can use "Remove Scurvy" and remove those crowd control effects.
BE AWARE WHEN PLAYNG AGAINST HIM!

You should always focus on items with Critical chance, and Critical Damage, because Gangplank is all about criticals, if you complete the build your critical should be around 1000, and every 4 seconds (the time of the cooldown from Parrrley) you can try to critical again. So you should start wth avarice blade for gold and +12% critical chance, then you get Sheen, so your attacks will deal the double after using a skill, then you get the boots, make an Infinity Edge, which makes your critical deal 250% instead of 200% your normal hit from the skill.. Then you get Trinity force, so you can get more movement speed, attack speed, critical chance, attack damage, HP and Mana... Then you get a Phantom dancer, so you get more critical chance, more attack speed and more movement speed.. Then you get Last Whisper, so you get more attack damage and armor penetration, which is very important for GangPlank, because Parrrley is based on Physical damage, and walks along with armor penetration... Then you'll get Atma's Impaler, so you get more armor and more critical chance, and also hit +2% of your HP on your physical attacks.

You should use "Exhaust" on enemy champions everytime they're with low HP, so then you can kill/gank him.

You should use "Ghost" to escape from enemy Ganks or when you're at low HP.

Always focus on Parrrley, because it's your most important skill to use. Then you upgrade Remove Scurvy, and your Ultimate, because Raise Morale could be the last one to be upgraded, but you need to, at least, unlock it on mid game.

Well, i did good making this build. I did 30/11/12, with 19.7k money, and a complete build, then i sold my boots, and i was going to buy another phantom dancer, because of the critical and the attack speed, but they destroyed the enemy nexus before i could buy it. Even though, i killed a small quantity of minions, 89 minions, while the others killed 150+ minions. If i had killed more, i would have bought the Phantom before the game ended, but i'm so bad at killing minions, i always kill a small quantity, no matter which champion i'm playing with. So good luck playing with G.P. and "Critical them!".