Here is how you balance Gangplank without completely nerfing him with the nerf bat
The current nerfs on the PBE is just 10 damage nerf in his Powder Kegs on all levels.
Armor penetration (Total armor) is still 40%, just like live, and bonus damage on champions is 50/80/110/140/170.
But firstly let's talk about the passive.
Currently 
passive is a "free" true damage auto attack over time, that refreshes everytime you kill a barrel, or has a cooldown of 15 seconds.
What Gangplank needs a nerf, it's on this one. An instant refresh isn't the way to go. Maybe a reduced cooldown of 5/7.5/10/12.5/15 seconds, depending on the Powder Kegs level is the right way to balance the passive in his laning phase.
Now onto is Powder Kegs, where riot has a difficult way on balancing it.
The Powder Kegs have a 40% total armor penetration and has a static damage to champions of 60/90/120/150/180.
Instead of having a simple static damage to champions, how about having a total AD or bonus AD or even total crit AD?
Firstly, Total AD would be great for all phases of the game. Let's say that you start the game with 70 AD (counting runes). If you have a total of 0.35 total AD, you would have only 28 bonus damage to champions. If you buy, let's say, an Essence Reaver, a Yoummus Ghostblade, 2 Infinity Edges and a Lord Dominik's Regards, you'll be getting at level 18, a total of 107 + 65 + 65 + 65 + 65 + 40 = 407 AD.
407 * 0.35 = 142,45 bonus damage to champions late game or at level 18.
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Let's imagine that you are level 9 with 2 and a half completed items. An Essence Reaver, An Infinity Edge and a Giant slayer (from LDR). You'll have 76 + 65 + 65 + 10 = 216 AD
216 * 0.35 = 75,6 bonus damage to champions at level 9. At level 9, you are expected to at least have 2 points in Powder Kegs to do 80 bonus damage to champions.
TL;DR: Basically, 0.35 total AD is enough to slow him down early game, and have the same impact late game by having him to buy one or two of the armor penetration items, instead of just having him to get full critical strike and have AoE crits everywhere.
Onto Bonus AD. ON this hand, this is a complete nerf early game, obviously. Let's say, he has 0.5 bonus AD. From runes, you'll have at least 15 AD, leading you to only 8 extra damage to champions. But in late game, having the full build above will get you 150 bonus damage to champions.
In level 9, with the items above, you'll have 70 bonus damage to champions.
So here's my suggestion for his passive and E balance. Thanks for reading.