Landing barrels while laning is a great way to poke without using mana. It takes practice but once you get the hang of it barrels could win a lane for you. Be sure to use you Q on minions as much as possible to get your 500 Silver Serpents to upgrade your ult.

Gangplank could alone win a team fight with a good ult so be sure to position it right in team fights to take some of the other team's focus off of your carries and more on getting out of your ult. While fighting also try and land as many barrels as you can on their tanks and carries because they have a lot of armor penetration. Shoot Qs mainly at carries to crit them down.

Gangplank full build 100% crit is a monster. You'll enjoy 1 shotting carries and burning through tanks. Gangplank late game is an unstoppable force.