Based on the famous manga Gambling Apocalypse: Kaiji, the 2007 anime series Kaiji: Ultimate Survivor centres around the protagonist Kaiji Itō, a compulsive gambler who owes money to loan sharks (sounds like Gi-hun, right?). In an effort to pay off his debt, he participates in a mysterious gambling event on a cruise ship. When he regrets his decision, it's too late to turn back because he's already wrapped up in a series of deadly mind games where it's every man for himself.
Watch Kaiji: Ultimate Survivor on Crunchyroll.