I finally had the enlightment to where Gangplank's ship shoot the rift from:

It's not a normal ship.

It's an AIR-ship.

The Bilgewater event has left even hints with Graves and Twisted Fate going off to Piltover and now that the harrowing is over our dear pirate might have taken a little holiday trip from fighing against Miss Fortune to pay the city of progress a little visit. After all I'm sure he wants TF and Graves as much dead as he hates MF...

And then he steals the first zeppelin he could find, loads it up with a shitton of cannons and flew back to Bilgewater with his new supreme ship - after all most of Bilgewaters canons are made to hit targets below or on the same level as them. The airship would be like the ultimate weapon over here.

Also it is canon Piltover and Zaun posess airship technology, you can actually see one in Jinx's music video and one in Corkis splashart. And old lore even had an airship race between Piltover and Zaun...

Finally that mystery is solved.