In this paper, in order to overcome the self-excited oscillation in the high-gain relativistic klystron amplifier, an X-band high-gain relativistic klystron amplifier (RKA) is designed driven by the relativistic beam with its current at a kA-level. The corresponding diode impedance is 800 Ω. Its output power reaches 284 MW at the frequency of 9.47GHz, and the gain and efficiency are 50.6 dB and of 37.4% respectively when the beam voltage is of 800 kV. And for the obtained GW-level RF power in low-impedance pulsed power sources, the power microwave combination technology of off-axis eight-tube high-gain RKA is used. In the three-dimensional model, the microwave output of this kind of RKA is almost constant when the device is at off-axis 54 mm. Simulation is based on the 4.5 T superconducting magnet which is 4.5 m long in the laboratory, and the eight-tube high-gain RKA is simulated using a magnet with the microwave output of 284 MW. Then an eight to one power combiner is designed using HFSS software, and the combiner with eight-tube high-gain RKAs is simulated by a three-dimensional model. Its output power is 1.84 GW, with a gain of 50.7 dB and efficiency of 28.8% respectively.