An employee was moving a mortality from a farm using a tractor. The employee pulled in toward the pad and backed up the tractor. The tractor bucket became caught on the gate. The employee exited the tractor and was attempting to disengage the bucket when the tractor rolled forward and pinned the employee's right leg between the bucket and gate. The employee's right lower leg broke. The tractor was not shut down and the brakes were not set at the time of the incident.