I'm not a Gangplank (GP) player, ... but to my knowledge the biggest strength that he offers his team is his ability to split push and still provide assistance to his team with his global ultimate.
From what I have seen, he basically just plays cat and mouse, forcing his lane opponent to babysit him, not allowing them to make plays with their team.
You either focus more on damage like an ADC, shoving waves in as soon as the enemy leaves, or you focus on being a bruiser that can't be forced out of lane.
Again though, you MUST keep an eye out for the best use of your ultimate to help your team across the map.
If you get ahead, the enemy will need to deal with you, which takes pressure off of your teammates elsewhere.
If you are behind, you should still be able to at least hold a lane that is the least priority.