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.
This build is probably one of the most effective builds for Gangplank. The 2 Brawler's Gloves are quick efficient items to get crit early in the game along with the 2 Cloak's of Agility. By then you should have about 50% crit and well off in game depending on your skill/team. The Ninja Tabi's are helpful with survivability and also movement of course. When you buy the Infinity Edge next your crit chance should be about 60% and you will be critting 800+ damage with Parrrley. The 2 Phantom dancers afterward will make your crit so you crit practically every hit and the movement speed will help with chasing and getting away. By the time you get the full build you will be unstoppable. The game is usually over before you finish your build completely, usually from you destroying them :P. But thats my advice on this build, gooluck with it and I hope you all have a lot of fun with Gangplank now. :)