Well I don't play Gangplank but I have fought against him. The Gangplanks i crush are the ones who are bad with their Barrels and placement (as in where they are fighting me). The Gangplanks i don't crush are the ones who place good barrels that actually hit me, they also know when to heal rather then omg im low on health time to spam heal and this random moment. The Gangplanks that crush me are the ones who place great barrels that can be used offensively or defensively, they know when and where to place them and where to stand, they also know the right time to use their heal.
So overall I'd start with barrel placement, when and where to use them, and how to use them in a manner that is flexible for both offense and defense at any given point. Then practice on when to use your ult and heal.