Why not put a mana cost on GP's Barrels?
GP is really strong. I'd say broken, but apparently some people have a lot of trouble playing him, so newcomers make him seem less unfair. In my opinion, as well as that of many other people's, he needs a nerf.
I suggest starting with making him less abusive in lane for someone who hyperscales into mid-late game. This can be done by putting a mana cost of some kind on his barrels. This way he doesn't just get to freely spam them without end.