Gangplank is a carry and a very useful champion. He has one single target nuke which is his q attack. He has a cleanse utility and heal as his w and a team utility for team fights as his e. His ultimate can be the turning tide between losing a teamfight or winning one. His passive is a DoT and also a stacking slow.

Gangplank only has one single target non-skillshot nuke, so he needs to be able to buff that as much as possible. Parrrley is affected by critical strike chance and also sheen but sheen is not included in this build. Armor penatration marks and quins will work very well to help penetrate early game armor of champions. Crit chance glyphs are just mostly icing on the cake for your crit chance. You could go furor instead but those are very costly, so crit chance is much more cost effective.

There are two ways to build gangplank, a melee damage carry, or the old build with avarice and trinity. This build here for mastries is to focus on ranged harassment against other player, but to also get key last hits on minions with parrrley. It has a very good bonus amount of gold per kill and the gold goes up by 4/6/8/10/12. The crit damage buff in the offense masteries is key to gangplanks early game kills. If you manage to get some early game kills or atleast make people recall due to an insane parrrley critical, then you can get ahead.

It may be bad to start off without any damage at all at the beggining of the game but it works. Once the phantom dancer is up you have about 35% crit chance with runes and masteries. The attack speed will be fine because the grog soaked blade ability can stack up to some pretty strong damage if you get to five stacks. The infinity edge is key for the crit damage bonus and the crit chance. Your parrley crits should take out a scary amount of health, so be ready to chase people.Cooldown boots are to help get his parrrley off cooldown faster, but also getting blue buff can help make it even faster. A bloodthirster for the aditional damage and lifesteal but also in this build you may have to get an exacutioners calling if there is someone like tryndamere or soraka. Another phantom dancer for that 90% crit chance and fast attack speed followed up with a last whisper is a deadly force.

Ghost and exhaust are used in 2 different ways for gp. You can use ghost combined with raise moral for a rapid escape or to chase after another champion. Exhaust can be used to escape from other champions, slow down other champions, or to reduce the dps of other carries that may be doing severe damage to you. It all depends on the situation.