starve him. If you can zone him to the point where he is using Q to farm, he wont be hero harrassing. Once his hp is at half, you can likely dive him and kill him.
He is a squishy bug.
I myself was just able to build a trinity in a game against 4 tank's. They surrendered before I really got to build much else.. I had tier 1 boot and a tear. I was able t take out their health in decent chucks considering how much hp they had, but if they had rushed us i wouldnt be able to deal enough dmg fast enough.
Yes GP can deal high dmg in a single hit, but in melee there are not alot of situations plank is comfortable in. Use this against him!
I was able to do so well because I was allowed to just spam my Q and proc sheen all day long. Which basically means I get double my AD in every Q, and 4X my AD in a crit. X4.5 my AD once I have IE. This is huge damage indeed. BUT when you factor in the cost for these item's and that they give him no endurane, it is a risky build.
Higher risk = higher reward.
GP does well if you don't shut him down. TO do so, outrun him, and outrange him.
garen, mundo, nid, anybody with a speed burst can be a tricky target for plank. Match them agaionst him in early lane, and he will have a slow start. Hopefully your mid lane can dominate him during teamfight's and keep him out of the fight.
In proper positinoing your tank will be between you and him landing a CC on him. Yes he can cleanse 1, but he can't cleanse 2.