B737 Captain reported while pre-flighting the aircraft on the ramp; the Gate Agent mentioned the main landing gear brake was on fire with flames present. The Captain initiated a gate evacuation of passengers deplaning; notified the Flight Attendants; and Fire Department. The Fire Department action was to let the fire burn itself out. After the fire extinguished; the Fire Department allowed crew and passengers to re-board the aircraft to collect their belongings.

Narrative

While parked at the gate I commenced a post flight walkaround. Just as I was inspecting the nose gear; a Ramp Agent mentioned casually that the left main gear brake (#2) was on fire. I looked over at it from my position in front of the aircraft and did see a small flame from the inboard side of the brakes. I told the Ramp Agent to get all Personnel clear of the aircraft immediately. I then ran up the jet way stairs into the cabin; stood at the flight deck door and told the First Officer that we had a gear fire and to call the Tower and have them roll the fire trucks to our gate. I then turned to the PA system in the forward cabin and made a PA to the deplaning Passengers to make an orderly evacuation out the L1 door only; and to leave their bags on the aircraft. I repeated this command about three times during the evacuation.The Passengers evacuated in a calm and smooth manner and no injuries were observed during or after the evacuation. The Flight Attendants assisted with the evacuation by telling the Passengers to leave their bags and calmly exit the aircraft. After all Passengers were off; I ordered the Crew off the aircraft; and I did a walk thru to ensure all Passengers were off the aircraft. After exiting; I went outside the jetway and saw a Ramp Agent walking away from the still burning brake with a handheld fire extinguisher. I told him to stay away from the aircraft and that it was dangerous for us to attempt to extinguish the fire. The Fire Department arrived around ten minutes or more later. I can't exactly recall but I believe they approached the wheel and took a temperature reading. They retreated about 20 yards back and simply waited for the fire to extinguish. After 30 minutes or so; we were cleared back onto the aircraft to retrieve Passengers' bags and bring them up the jetway.During the 30 minutes or so that the fire was allowed to burn itself out; I spent time up in the terminal addressing Passengers on an individual and small group basis assuring them that everything was alright and expressing my appreciation for their orderly evacuation. After the Fire department cleared us back on the aircraft; I began making phone calls to Dispatch; the Operations Control Center and Maintenance Control.