Gangplank was born the son of the dread pirate captain Vincent the Shadow - one of the most wealthy and feared pirates in all of Blue Flame Island. One might think this would have spoiled the boy with a cushioned life of privilege, but the truth is quite the opposite. Growing up in the city of Bilgewater isn't easy; pirates are not known for their compassion, and that most certainly extends to their families. Vincent wanted his son to grow up tough and strong, so he was extremely hard on the young Gangplank. Even as a child, Gangplank was as mean as a snake and is said to have slept with his eyes open. As he grew, the young man rapidly became the most ruthless and feared pirate in all of Bilgewater, and his daddy was never more proud than on the day of his son's eighteenth birthday - when Gangplank stabbed his old man in the back and claimed the famed pirate ship, the Dead Pool, for his own.
The continent of Valoran, however, is a dangerous place for pirates; Gangplank could read the writing on the wall. The den of pirates known as Bilgewater would soon be pulled down by the undertow of Valoranian politics and the Institute of War. It was time for Bilgewater to have their own champion to represent them in the League of Legends, and who better than the fiercest pirate of them all? It is said that Gangplank has enough power and favor to claim the title of the Pirate King back home, but that he is simply biding his time and building his reputation as a champion before he returns to a life of piracy.
Source: The Game Website.