Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
In general :
The Pirate is a good late game carry who can beat them all, but u need some tanks to attack at the right time. Don't initiative the fights, cause Gangplank is rly squishy. Using ur ult to get control about some hard lanes at the beginning and use it to win team fights. Use Parley to get creep kills, when ur lane is hard-fought. Get a lot of money to become strong.
The Strength of Gangplank is his ability to crit by shooting his Parrrley, so the damage will be multiplied. Gangplank has two possibilities to buy Items, as a tank with Atmas Impaler or playing him with pure Crit Items. The second Alternative is the best to deal with, but I prefer to use Defense Abilities and some usefull runes instead of pure Crit Runes.
Summoner Spells :
Ghost Walk is important to hunt or to get away, Exhaust is an option to slow an opponent or win fights against twitch or other hard opponents.
If the team has one or two exhaust, u can also pick Ignite or Flash to be more flexible.
Optional u can use Health Quintessesences or Cooldown Glyphs
If u have a solo lane, u could learn Raise Moral first to deny creeps, so that the opponents don't get the full experience like u.
But I prefer to have Remove Scurvey more effective.
My Items Line :
At the beginning u have to buy Brawlers so that u have a chance of around ~25% to hit with a Crit. Than u need a second Brawler's and upgrade one to Avarice to have a good chance to Crit. Phantom Dance, Infinity and Bloodthirster are necessary.
Than u have some options, you could buy Banshees or some Magic Resistence or u could buy my suggestion of items.
So Take The Pirate and own them!!!
P.S. I'm not a native, so my english isn't so well