Early on its hard to farm gp so you got to get a phantom dancer it helps with citing off your Parrrley. Try last hitting with Parrrley because you get extra gold. When you get your 2nd rush infinity edge to max crit and have lot of crit dmg for extreme power. then get all bloodthirsters to max damage to instakill the creeps.

your runes and masteries should be 21-0-9. yours summoner spells should be Flash and Exhaust.
Your skill Sequences should be Q,E,Q,Q,W,R,E,Q,Q,W,R,E,E,W,E,R,W,W,E.

TEAMWORK TEAMWORK TEAMWORK is main objective for gangplank. You NEED to stay behind untill someone is low and hit the with parrrley. When in a team fight the attack you first so try to stay back like i said.