A moving-base rotating accelerometer gravity gradiometer (RAGG) is an instrument for measuring gravitational gradient signals produced by geological bodies with a certain signal bandwidth. Development and improvement of RAGG requires that they be subjected to testing and calibration; however, the zero-frequency gravitational gradient signals produced by static test masses are not suitable for this purpose. We propose a method in which multiple test masses simultaneously rotating about a RAGG at different angular velocities and in different circular orbits produce the multifrequency gravitational gradient excitation required for testing or calibrating the RAGG. We also present a gravitational gradient extraction method that combines a fore-end circuit design, a multirate filter technique, and a quadrature amplitude modulation demodulation technique. We describe in detail the procedures for gravitational gradient extraction. Multifrequency gravitational gradient excitations are applied to evaluate this extraction method. A RAGG physical simulation system substitutes for an actual RAGG in a multifrequency gravitational gradient extraction experiment. The extracted multifrequency gravitational gradient signal is consistent with theoretical predictions. The gravitational gradient extraction error approximates the noise of the RAGG physical simulation system. These experimental results suggest that the proposed gravitational gradient extraction method is feasible. The research presented in this paper is of great significance for engineering applications.