I appreciate Gangplank nerfs and we all agree something needs to be done..
But I still completely disagree with these nerfs. You're just going to keep nerfing him until his cooldowns are too high, his mana costs are too high and his base damages are too low. You'll basically be doing exactly what you did to old GP during meta golem where you kept constantly nerfing him. The difference is, I don't think he needs these nerfs. It won't change anything, he will still be picked in almost every competitive game.
I play GP and I can tell you the biggest problems with him are his bonus gold passive on his Q and his Q applying melee bandit gold. With these two things, he farms up so much gold just by existing. Even if he's getting zoned for a wave or two, he's still collecting some gold. And that's rare a champion can do that to be honest. He can sit back and just keep a barrel behind him and clear waves this way and not interact. There's nothing that can be done about that, GP will be a safe farmer no matter what. GPs ult will provide great zone control in teamfights no matter what. GP basically already has a free ticket to late game but I don't think you should complete remove his ability to have lane presence and make him a non interactive farmer either because that playstyle is extremely unhealthy.
What needs to be done is removing his bonus gold passive on Q and changing how his Q works with masteries like grasp and bandit, these are clearly ranged spells granting melee benefits and this shouldn't be the case. His Q's gold passive is what makes him impossible to shut down. Even when you have large leads by 2-3 kills on him, at best you're slightly ahead in gold. Between his barrels, ult and orange, snowballing on GP is almost impossible. And that shouldn't be a thing for a champion that scales so hard into late game. And these nerfs don't change that, they're just going to force him to be more farm heavy and non interactive and players will feel like they can't do anything to stop the GP train from going full force late game.
Make him able to actually lose a lane significantly enough that he has to consider something other than farming two waves to make back up whatever deficit he had. This would be a much better change than what you're doing right now.
1200 damage Q's every 3 seconds.
. You might say that TLD is what makes champions suffer nerfs and not the champions themselves. Why do i mention the pros you say because riot nerfs champions that mostly does well in Pro matches. I am only a silver player but i have watched alot of HIgh tier matches meaning Masters and Challengers matches and until the bad LCK Gangplank came up from the dephts almost nobody played GP unless they mained him. Nerf TLD or replace it before you start nerf every champ that synergize sometimes all to well with it thats the simplest thing to do or you might find your own champions being nerfed to the Kraken just because the early TLD dmg was too much. Btw i watched the semi between IMT and C9 sorry but the only impactfull Gp there was Hunis whos mechanism on the champion was good while the player called Impact was used as Ye old Ult bot and did not make an impact so to speak.