I think it is because why he carries super-hard himself, at the same time it is extremely hard for most toplaners to make any aggressive plays on him to shut him down.
If you go at him, he will just pop a barrel, applying it's extremely heavy slow and gaining MS himself. Even if you manage to somehow root him, he has remove scurvy, and yes it has long cooldown, but for most toplaners getting close to GP more often than 22 seconds while maneuvering between barrels is simply not realistic.
And even if EVERYTHING finally goes wrong for GP, many champions will be forced to forget about fighting him, due to absurd damage (and another slow) of Cannon Barrage.
So, he carries hard, and many champions can't realistically do anything about him, because he is just too safe. GP is basically AD Azir at the moment - yes he takes skill to play, but this doesn't nearly make up for strength he gets and terrible gameplay for his enemies.