Coupling moments, nonlinear friction, and unmodeled dynamics between inner and outer gimbals are the main factors that influence the precise control of a double gimbal control moment gyro (DGCMG). To improve the interference suppression ability and ensure the output angular speed accuracy of gimbal systems, this study presents a composite decoupling control method based on a nonlinear cascade extended state observer (NCESO) and sliding mode control. All disturbances in a gimbal system are regarded as the lumped disturbance, which is estimated by the designed NCESO. By combining a sliding mode controller, these disturbances can be eliminated from the output channel of the system. Finally, simulations and experiments of the control method proposed in this study and a linear cascade extended state observer (LCESO) combined with state feedback were carried out. The simulation and experimental results show that the proposed method has better interference suppression performance and dynamic response performance. The angular speed fluctuation of the inner gimbal is reduced from 0.5 (°)/s to 0.2 (°)/s and the angular speed fluctuation of the outer gimbal is reduced from 0.45 (°)/s to 0.15 (°)/s. Moreover, the speed tracking error is reduced from 1.8 (°)/s to 1.2 (°)/s and the phase delay is reduced from 8° to 1.3° when tracking the sinusoidal reference signal.