what is the significance of Gangplank's knife?

so he can bring a knife to a gun fight AND bring a gun to a knife fight.

Might've been a red herring, but it did glow when Fate cut himself in freeing Graves.

It's very likely rare and valuable, since it was found in Gangplank's warehouse, but magical properties are hard to figure out, since it's only described as having a magical effect once, really late into the story. As for its significance, Sarah Fortune used it as a larceny mark for Twisted Fate to put him on Gangplank's "red list", alongside Graves, who she tipped off about Twisted Fate's impending location. Having Twisted Fate steal the dagger and having Malcolm start a huge gunfight with a heavy toll on the docks raised Gangplank's ire enough to stage a public execution of the two former partners - allowing her to destroy the Dead Pool and kill Gangplank in one fatal barrage from the Syren.

Seeing how the Black Market items are supposed to be from GP's warehouse, I would most definitely say it is magical, and may be related to how GP comes back.