On October 26, 2014, about 0830 Pacific daylight time, a Cessna TU206G, N4776U, veered of the runway and collided with a ditch at a private airfield in Monument, Oregon. The pilot/owner was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The commercial pilot, the sole occupant, was not injured; the airplane sustained substantial damage. The local personal flight departed from Monument Municipal Airport, Monument, Oregon about 0815 with a planned destination of the airfield that the accident occurred. Visual meteorological conditions prevailed and the pilot had not filed a visual flight rules (VFR) flight plan.The pilot stated that the airplane touched down on the gravel runway at a slow groundspeed; his farm was near the runway and he had landed there regularly. Immediately thereafter, the airplane began to veer to the right (north). The pilot used both the brakes and aileron inputs in an effort to return to the runway center. Despite the pilot's attempts to regain directional control, the airplane continued off the right side of the runway and collided into a ditch. The airplane came to rest inverted incurring damage to the wings and tail section. The pilot further stated that the loss of directional control was precipitated by a landing gear malfunction. He opined that a brake likely seized after touchdown. According to the recovery personnel, the left tire made a long track in the gravel angling to the right and then the track dropped off the edge of the runway after the airplane's right wing tip collided with a tree. Indications on the runway (skid marks in the crushed rock) indicated that the left brake was working, but there was no traction due to the looseness and depth of the gravel surface. The nose wheel rolled but still dug a trough, the right wheel only touched down a few times, appearing to have been in the air most of the time. A Federal Aviation Administration (FAA) inspector looked at the disassembled airplane following the recovery and examined the brake system. He noted that they were both intact and both tires turned freely. He was not able to operate the brakes with the pedals, and it could not be determined if that was a result of the accident and/or subsequent recovery.