Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Gangplank has a very wide range for a melee. To start off the levels grab RM for Deny, because this will prove to help you if you are laning against ranged. As you level depending on your lane you will be at 1 of 2 crossroads.
1: You are forced back on your turret. If that happens continue using RM to kill minions and deny xp(be aware of the cannon minions to have better results). Now you may be low on mana most of the time, but you have the level advantage and a partner to hold the lane. Once you hit level 6 start looking across map for the best use of your ulti. You need to remember that it isnt meant just for kills, but to also aid your teammates. Coordinate with them to set up a gank on an over-extended enemy champion, because they will be slowed and still able to be stunned or cc'd otherwise by your teammates. If you get the kill or only assist that is good becasue you just want that gold. After you have the money for it I strongly reccomend buying executioners calling because lifesteal stacks with parrley to create a very strong offensive heal. From there continue to build your crit and continue to set up ganks.
2. Your controling the lane and are about to destroy the turret. With this scenario you have lane dominance and are again continuing to use RM.(denying xp is very essential to helping out) Presumably you are with a surplus of gold and complete control of the lane. Deal as much damage to enemy champions and when you run low on mana recall and buy some much needed items. Stick to the item sequence because that is your best shot at surviving. Again coordinate with teammates to gank over-extended champions.
Now its getting late game and teamfights are becoming more and more intense. Try to stay out of sight, but still in range of the actual fight because you will need to catch runners. With the ulti over the teamfight there will be some who will immidiately run. IGNORE THEM! Do not waste time chasing immediate runners because this wll get you killed easily, instead wait for a squishy champ to come running by trying to escape, ecause they will be easy for you to 1 shot with parrley. After he is kill you are going to be the prime target for any remaining champions in the area. If you are well fed and they are low health you can try to mop up whats left because you have Remove Scurvey and Parrley stacking with lifesteal those hard crits will help keep you alive. If you cannot stay alive remember minions can still be your savior, quickly run throught them and if you are still pursued quickly RM 1 and start hitting that enemy champion with all you got because once Parrley is off of CD free heals, by the time the enemy champ realizes tht he can't kill you it will be too late. On a last note should you enter a teamfight actually with your team use RM to help your melee champs and focus your attacks on thier target, because your passive reduces healing recieved(feeling extra annoying throw ignite on him too) and show them why you are "The Saltwater Scourge".