First off, you must witness the Private Action in Kurik where a woman named Philia is warning the citizens to evacuate the city. Next, you must reach the final save point in Phynal. After that, you must leave Phynal and view a Private Action in Centropolis. You'll find Philia again in the lobby of the main building. After you talk to her, Gabriel's limiter will be removed and he will be much more powerful than his original form. It is recommended that you go through the Maze of Tribulations and get your levels very high (120+ at the very least) before attempting to fight him.