PROS

Very versatile champ and is able to build many ways hence the "Critplank" and "Tankplank" variations.

Free Quick Silver's Sash that also restores health.

Global ultimate able to pressure all lanes.

Very hard to counter pick, even against his hardest match ups he still does well.

Generates gold very fast through Parrrley and Powder Keg.

CONS

Weak against most ranged champions because of his Powder Keg mechanics.

Not very good in most team fights, he needs the support of his team if he wants to do some damage.

Gets focused in team fights, because of his gold advantage and high damage.

Early game is all about harass, whenever you get in range of your opponent just send a Parrrley their way. Even though you don't generate gold from last hitting minions with your Q, you still get the money from the Bandit mastery. As all lanes go, you also want to hit lvl 2 before them, because it gives you your Powder Kegs, a hard hitting but hard to land, ability that is useful for zoning and even escaping from junglers. When you opponent decides to back or leaves lane, you can also farm with your Powder Kegs by whittling the minions down till their low and then detonating a Powder Keg with Parrrley, granting you bonus gold AoE. On your first back buy Sheen as it almost doubles your damage with your Q, and from there you go to mid game.

Mid game is where team fights start to happen and lanes will start to lose or win. This is also the time that you hit 6 and get your ultimate, Cannon Barrage, an infinite range ult that applies AoE damage and slow for a period of time. With that in mind, you want to look for lanes where Cannon Barrage can get you an extra kill or even an assist. However, you might want to keep it for yourself if you know you need it as a finisher against your lane opponent. On lane, you want to be farming with Powder Keg now, as you have increased damage output, thus increasing barrel damage, maybe enough to 1 shot minions. By now you should have a bunch of gold from your Parrrley passive and should rush Trinity and IF, giving you a huge power spike. Something to note is that Gangplank is not a great ganker, though he has a long range AoE slow all he can really do in a gank is Q a champ and maybe get one or two basic attacks in. So it is best to stay top and just farm, and come when you're rich and have more gold than your ADC.

By late game you should be almost complete with your expensive build, and should start to help with tea fights more often, or you can split push top using your passive + reapplication of your passive with a detonation of Powder Keg. In team fights you want to stay back if you have not got your tanky items yet,(eg. Spirit Visage, Deadman's) and place barrels in a range that you can protect until it is ready for detonation. However, if you are tanks when the team fight starts you want to be able to lower their carries with well placed Parrleys or Powder Keg mechanics then engage on the carries when they are low enough. With your Remove Scurvy you are able to do this and hopefully get away with it as well.

This is my first guide, I picked Gangplank because he is my favorite champion right now, and used to be one of my favorite champs season 4(because I played unskilled champs). Personally, I think I could work more on my Early/Mid/Late game chapters, but I don't really have much to add except from personal experience and play style. I'm also skeptical if my guide is a little too wordy, I guess some people like simple, short guides, but mine is really wordy. I also did not know how to use many features on this site so excuse me if this guide is a little bland. Anyways have fun with Gangplank and hope you enjoyed my guide.