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So when GP detonates a barrel, it applies the damage of the attack + bonus physical damage in an AOE. This is a cool mechanic, applying your attack damage in an AOE by setting up your barrels properly. What isn't cool is the fact that this splash damage ignores 60% of TOTAL armor. This means that if Gangplank runs 21 offense masteries (for 6% armor pen) and buys a Last Whisper, his explosions have 101% armor penetration. They deal true damage every time.

Imagine if there was an AD carry with an ability that converted all damage applied by their next AA to true damage. It makes no sense, because the counterplay to AD carries is building armor. You might argue this by saying "But GP isn't an AD carry, he's a top lane melee bruiser!" True, but Powder Keg's placement range is 1000 and Parrrley's range is 625, so this damage can be dealt from higher range than most AD carries can autoattack. Building armor against GP, who is currently most viable when built as an AD carry, does effectively nothing.

I really feel like this is overkill, considering that the champion already has two other instances of true damage built into his kit (passive, R). I understand that the primary counterplay is to destroy the barrels before he pops them, but the punishment for failing to do so should not mean taking 1300 true damage every time.