Per the FAQ:
Q. Will I be able to buy tickets at the gate?
A. There will be NO TICKETS SALES at the gate after 11 pm Thursday, September 2nd. Tickets will be available at the box office at the front gate during the event, Monday through Thursday, August 30 — September 2nd. Tickets purchased at the gate will be higher than any previous levels and will increase over the course of the event.
Q. Are the gates to the event open 24 hours?
A. Yes, however, the gates will be closed to incoming traffic after 6 p.m. on Saturday, September 4th, prior to the burn of the Man.
So.. get your ticket ahead of time and don't try wandering in an hour before the man burns.