Riot if you were to nerf GP... no mana cost is kind strong...
It should be adding a mana cost to his Powder Keg. Just a suggestion but 40/45/50/55/60 mana.
The current lack of mana costs on Powder Keg means that GP can freely use it for zoning against other melee indefinitely.
This would make him more clearly limited early game. He would still able to do everything that GP can do currently early but whould have to go B at some point if he wants to keep zoning the lane.