Good question. We have a lot of debates internally about the power of gangplank. We think a lot of the current complaints about Gangplank are to do with "frustration", which doesn't necessarily track to "power". This isn't to say he isn't powerful, but he definitely has weaknesses. Pros are generally mixed on Tri Force build (which is more consistent, but lower damage) and the ER -> Prowlers (higher damage, but very squishy). "Strong laning phase" is insufficiently specific of a statement, though I agree in certain situations, he is strong for the entirety of laning phase.
He has a strong laning phase 1-3 for sure, but he has a very noticeable power trough and is susceptible to be snowballed against, especially in the Level 6-11 area in matchups where he has to use ult defensively. He also has a good number of decent counterpicks, is quite lacking when behind (cannot utilize vision differentials to get barrels off), especially with the squishy build .
He is one of the most unique champions in that if a Pro played 1000 games of Gangplank, he would theoretically always be overpowered if he's even remotely close to being playable for regular players due to his skillcap. (there aren't many champions in the game like this) So in this sense, the arguments for Gangplank being overpowered are more about "what is your definition for overpowered". Hopefully that helps.