Gangplank and Zed are rivals?

Because Gangplank has stolen some stuff from the Order of the Shadow. Fucking badass.

Having grown rich preying upon the trade routes of the Twelve Seas, Gangplank has made himself many powerful enemies. In Ionia, he incurred the wrath of the deadly Order of Shadow after ransacking the Temple of the Jagged Knife, and it is said that the Grand General of Noxus himself has sworn to see Gangplank torn asunder after the pirate stole the Leviathan, Swain’s personal warship and the pride of the Noxian fleet.

this

Gangplank stole from Zed,I think he stole a special dagger or something from him.

He also stole swains ship,which is why Swain is listed as a rival.

Gangplank stole some of his shit.

lol maybe lore update on Zed's part coming later.

Gangplank stole stuff.

also ninjas vs pirates meme

Because gankplank stole one of his ships.

Gangplank fucking raided one of Zed's temples