It is important to remember that his "q" ability, Parrrley, gives you extra gold if it's a killingblow. Farm as much minions as you can with it. With the extra gold you get from farming with Parrrley, you can buy elixirs that make you deal more damage. Once you have some crit chance, you are effective at harassing the enemies.

In late game, Gangplank deals loads of damage with Parrrley. He can even take down a squishy in 2-3 hits. Gangplank is very good at downing important damage dealers such as: Kassadin, Brand, Ashe and so on, wich makes it easier for the team to survive in teamfights.