The parasitic oscillation of the TE21-degrees gyrotron backward wave (gyro BW) mode is observed in a low-Q, 8 GHz TE011-degrees gyrotron. Although at low power (P(BW) < 5 kW), the oscillation of the gyro BW mode, simultaneously with the gyrotron mode, results in a maximum TE011-degrees mode efficiency of less than 0.25. The parasitic oscillation is suppressed by operating the gyrotron with a negative magnetic field gradient along the electron beam, which allows the maximum efficiency to reach 0.40 and the output power to be multiplied by a factor varying from 1.4 to 1.7. The optimum efficiency curve of the TE011-degrees mode indicates that the low-Q cavity behaves as a much higher Q(diff) Cavity. Excessive values of magnetic field gradient and alpha favor the TE012-degrees longitudinal mode, which oscillates in place of the TE011-degrees, mode and limits its maximum output power. This competitive process is responsible for the high-Q-like behavior of the optimum efficiency curve.