Try to gank as much as you can with teleport and using the homeguard boots. It will be an easy kill with a gangplank running like beast :)

Remember that the pickpocket mastery applies when u use parley to an enemy as an on-hit effect!

Early Game

To win early game try to poke them as much as you can without missing any minions. Try to use the bush to out-farm him by forcing him out of your minions. Winning early in the game is the most important thing in Gangplanks strategy because you can easily snowball them with the large amounts of gold you will have later in the game. Just try not to die and farm better than him. Killing is not as important as farm is. Try to last-hit some minions with the parley for extra income.

Mid Game

In the mid game try to help your teammates by casting your ultimates everytime they try something crazy. It's like you are in the team fights without being there at all, forcing one enemy to face you all the time at top.

End Game

End game try to stick together with your team while using your ultimate to either push another lane or to engage the fight.

Team Fights

In team fights try to bait a stun or something that you are able to cleanse with oranges :). After they focus you, you can turn around the fight and win it. Try not to tank a lot and prefer to be close to your carries to support them with the permanent slow you provide so they can kite easily.