Use your Powder Kegs with your Parrrly to Earn Extra gold and earn Silver Serpents Which you can use to upgrade you ulimate.

Put down your Powder Keg [E] at your minion so the if your opponent decides to go for CS you blow him up which ignores 60% of their armor.

Trading is pretty easy as Gangplank once they come into trade you hit them with an AA and apply your passive,put down a keg [E] detonate it doing tons of damage and reseting the cooldown on your passive,pop Remove Scurvy [W] and gain some health if you need some more damage pop your ultimate for the slow and damage.

Team-fights as Gangplank is very spooky for the other team because of your 1000 damage crits and your great AOE damage.

Use your powder keg plenty of times to put out your damage.Save your ult for people trying to escape because the slow is great and the damage is also good.
if you get focused down with CC use your Remove Scurvy.