Gangplank is fine right where he is.
I don't know what is with the boards wanting to change stuff that is fine and leave stuff that is not balanced. But I have seen several people in various boards discussions complain that Gangplank is broken and needs direct nerfs while others say if he is nerfed anymore he will be unplayable ane needs urgent buffs.
Truth is I hate playing against GP(Sorry Tidal Tyrant). That said Gangplank not only has clear counterplay but it is non-linear, it neither removes him from usefulness when countered nor makes hin completely noob stomp when not countered. And he also provides a lot of team utility, like Ashe levels of team utility with his global ultimate slow and dmg, his AoE slow for team fights. So he isn't a linear damage stick like Yi.
But that aside he is even well balanced right now. http://www.leagueofgraphs.com/champions/stats/gangplank/platinum
https://na.op.gg/champion/gangplank/statistics/top
http://champion.gg/champion/Gangplank/Top?
http://www.lolking.net/champions/gangplank?#/overview
No matter which crappy stat website you look at GP is fine. If he does ever become overbearing I am sure Riot will jump right on nerfing him. They know all about nerfing champions with team ultimates.
Oh, look, a happy little
BARREL
is that you?