Try to shut down Gangplank early with the help of your Jungler. Gangplank will eventually outscale you in the progress of the game, so putting him behind will make it harder for him to take over the map.
Don't engage with melee trades when Gangplanks Passive is up. It will help him win trades, so wait for him to use it on a minion before looking to fight.
Wait for Gangplank to blow up his Barrels E before looking to fight. If he has one placed in a nearby bush or has one under him, wait for it to blow before looking to fight.
In team fights, try to destroy his Barrels E before he can blow them up. This will reduce his AOE damage and his ability to win fights.
Try to avoid fighting in areas that are tight like around objectives or in the Dragon as his Ultimate R will hit your whole team. Fight in lanes rather than in the jungle and make sure you’re not grouped closely together.
Whenever Gangplank has his Ultimate R, ping your team so they know that he can impact skirmishes.
Gangplank will always be stronger if he has a Barrel E placed under him. Keep this in mind and try to fight him when there isn’t one.
At level 6, ping that Gangplank has his Ultimate R. It’s key that your team knows that he can use his Ultimate R to impact a fight.
Gangplank is rather weak in the early game and needs time to come online. Try and abuse his weak early game to get ahead.