In amplitude-modulation-type electroholography, the binary-weighted computer-generated hologram (BW-CGH) facilitates the gradation-expressible reconstruction of three-dimensional (3D) objects. To realize real-time gradation-expressible electroholography, we propose an efficient and high-speed method for calculating bit planes consisting of BW-CGHs. The proposed method is implemented on a multiple graphics processing unit (GPU) cluster system comprising 13 GPUs. The proposed BW-CGH method realizes eight-gradation expressible electroholography at approximately the same calculation speed as that of conventional electroholography based on binary CGH. Consequently, we were able to successfully reconstruct a real-time electroholographic 3D video comprising approximately 180,000 points expressed in eight gradations at 30 frames per second.