While on final approach to runway 10L at the Boise Air Terminal (BOI), the captain instructed the first officer to lower the landing gear. After the gear handle was placed in the down position, the pilots noted that the down-and-locked indication (green light) for the left main gear was not illuminated. The pilots completed the abnormal gear extension procedures but were unable to extend the left main gear. After confirming that the left main gear was not extended, the pilot was cleared to land on runway 10L. During the landing roll-out, the aircraft skidded off the runway, and came to rest on the north side of the runway. The aircraft received substantial damage to the left aileron and leading edge devices. PROBABLE CAUSE: “Failure of the forward gear door lock pin. An inoperative landing gear door and inadequate maintenance inspection of the aircraft were factors.”