He found it at the very bottom of the list:
When the image returned, CJ was no longer in Los Santos. He was standing in a grey, infinite grid—a void of green wireframes and floating text. This was the backdoor. The raw code of the game given form.
Floating in the void were ten giant, rotating orbs. Each orb was labeled:
The door opened to a single room. The walls were made of TV static. In the center of the room was a podium, and on the podium was a save icon—the floppy disk—pulsing with a black light.
He selected it.