Riot should smooth dmg for him. In early game GP is always pounded to the ground by anyone without any effort while from mid-late game he can obliterate literally everything.
For Q: they should make the same treatment as for Garen's E. They should increase base damage of his Q but capping his crit dmg (in result dmg with crit would be the little lower than live but less reliant from crit). It would scale with Infinity Edge as normal.
W is fine, maybe some changes in scaling numbers (mostly for lower ap scale) but it's fine as it is.
For E: Many times suggested - casting shouldn't interrupt moving!
For R: Instead for damaging hitting cannonballs, it should do damage over-time in zone. Simple and would do the job. Rest would stay as it is (duration, slow, OP laugh). I wouldn't mind for some low AD scale.
Passive is also fine, no reason for change (maybe some base dmg nerfs and added scaling).
Just a random thought as I look here, what do you think?