Gangplank is very weak and balanced champion based on skill, nerfs are not appropriate
I feel as though Gangplank is the best he has ever been since Season 8 and Runes Reforged as a healthy and balanced carry type champion in that his early game has finally been adjusted to where he is playable and not such a risky pick. There is also much depth to playing Gangplank and understanding his highly complex and intricate gameplay that is nothing like anything else in the game (of course, this is why he is classified as a Specialist).
Nerfing him based on mass community complaints is honestly not the right way to go about it. If anything, Gangplank is in need of subtle buffs to compensate for his success is almost entirely dictated by his incredibly high skill cap. In this regard, he is practically the same as Riven and there is a considerable difference between your average pick up and go Gangplank and legendary players like Tobias Fate.