Everyone has their own way of playing a champion and cannot be told which way is best for their playing styles. This is just my own build for Gangplank and for it works, for you it may not.

Pros
- High Damage Dealt.
- Great Mobility.
- High Critical Damage.
- Quick Cooldowns.
- Often Strong Self Heal.
- Great in 1v1 Fights.

Cons
- Low defense.
- Low Health.

This is purely a guideline and is not necessarily recommended to follow.

Parrrley, is great for dealing high amounts of damage, and slowing your target at the same time.

Remove Scurvy, is a great self heal ability. This allows you to solo your lane if you need to, allows you to remain in team fights longer, and lure an opponent into attacking so you can catch them unawares and heal and dish out some quick damage.

Raise Morale, is great for when you enter a team fight, as it boosts all your allies in a certain range's attack and movement speeds. Helps when fighting an enemy in a 1 on 1 fight so you can deal more damage faster.

Gangplanks Ultimate, Cannon Barrage, is great for getting running enemies, to get assists at another part of the battlefield or to slow enemies chasing a teammate ( you can waste it on this often with the cooldown reductions you have)

I choose the Greater Mark of Critical Damage for the crit damage increase, easy enough to see and understand if you play with a crit champion.

I choose the Greater Seal of Evasion because the hhigher your evasion, ability to dodge, the better your odds of surviving a 1 on 1 or in a team fight.

I choose the Greater Glyph of Attack Speed since they increase your attack speed allowing you to deal more damage as a result.

I choose the greater quintessence of desolation since it increases your armor penetration up to 30, after you buy Youmuu's Ghostblade.

Offense
Offensive mastery choices are the way to go considering this is a build for Gangplank which is purely critical strike oriented. As such magic damage is worthless, as is extra damage to minions, considering how you will be destroying them later on with 1 hit on occasion and 2 hits if you don't crit. Lifesteal isn't necessary since you will not be in fights long enough to bother with it.

Utility
Utility mastery choices are a good way to follow up your offense. Allowing you to revive faster, recall faster and move faster. These will change how your team, if aced, will not need to worry quite as much since your revive time is going to be seconds sooner, and those seconds will make all the difference.

Boots of lucidity - 15% cooldown reduction, allows you to get skills available faster, primarily your remove scurry.

Shurelya's Battlesong - health, health regen, mana regen and cooldown reduction, easily a good item to choose for faster skills to unload without using your mana as much.

Youmuu's Ghostblade - 30 attack damage, 15% critical strike chance, 15% cooldown reduction and 20 armor penetration. Again allows for fast skill unloads while increasing damage dealt.

Infinity Edge - 80 attack damage, 25% critical strike chance - Critical strikes deal 250% instead of 200% more damage. Definitely increases damage dealt.

Phantom Dancer - 55% attack speed, 30% critical strike chance and 12% increased movement speed. This will increase damage dealt, and your mobility.

Atma's Impaler - While you may not have that much health to be of much use with the passive ability, that's not why your even buying this. This is purchased for the 45 armor and 18% critical strike chance, making you less squishy and more powerful with critical strikes.

Heal - A precaution if you get to low health and your W is on cooldown, which still has a relatively long cooldown even with the cooldown reduction you are getting from your items.

Ignite - For those tough situations where an enemy runs away and is too fast to keep up with, mind you could always replace this with Exhaust, in order to slow down your target and get in range of Parrrley, which also slows your target. Ignite is more early game oriented whilst Exhaust is more late game oriented so that a turret won't slaughter you as easily.