I highly doubt that he's dead, but...
Well, I think it's too early to declare Gangplank dead and done. When a story paints someone's death as that certain, yet doesn't present the body, there's a good chance they're actually alive. Especially if they're a vicious bastard like Gangplank who can come back to inconvenience everyone for future drama (good guys have a harder time pulling the same stunt).
But if the Lore Team's playing for keeps, this'll be me when I find out... http://gifrific.com/wp-content/uploads/2012/09/joaquin-phoenix-shock-signs.gif It'd change everything.