When Gangplank triggers a critical hit, he will use his pistol for the autoattack instead of his sword.
The final boss of Donkey Kong Country King.K.Rool is fought on his ship, the "Gangplank Galleon". During the battle he actually makes cannonballs fall from the sky too.
Gangplank's skill Parrrley is spelled with 3 r's (Parrrley), a reference to popular conceptions of the way pirates speak.
Somewhat ironically, the term parley is used to describe a diplomatic discussion between enemies in regards to forming an agreement or truce. "Parley" is also a running gag in the Pirates of the Caribbean franchise.
His version of "negotiation" resembles that of Bruce Willis' character in the film The Fifth Element: as seen here:
The Pirate and Ninja buffs are corresponding references to the Pirates vs. Ninjas meme.
Gangplank was the only champion in League of Legend capable of denying, an act of killing a friendly minion to rob enemies of last-hit gold bonuses. The ability to deny allied minions was removed in v18.104.22.168.
His lore might be referred to the Dragon Highlord Duulket Ariakas from the Dragonlance Chronicles.
Both had fathers that is unforgiving towards their son, both stabbed their fathers in the back then claimed their prized possession and both their fathers were "never prouder than the day he was stabbed in the back."
NA user SantiagoBR gave birth to the "But I ate some oranges and it was k" meme. This was further popularized by mcr00sterDOTA's "This is why we hate Gangplank ". The thread can be accessed here.