This paper presents a novel method which is proposed to determine the optimal receiver coils when the transmitter coils are predetermined for multitransmitter wireless power transfer system, using the gradient-based optimization algorithm. The fixed grid-based coil representation, which is newly introduced, allows for effective changes in the coil layout during the optimization through assigning appropriate values for the reference coil turn and relative coil turns, which are set as design variables in the proposed method. The magnetic field strength, induced voltage, and coil mass are mathematically derived in order to obtain their analytical sensitivities with respect to the design variables. As an application example, a wireless portable charger with multiple transmitter coils is selected in order to minimize the mass of the receiver coils, while satisfying the induced voltage and magnetic field strength constraints. The optimized coil layouts are then experimentally validated, which demonstrates the feasibility and potential of the proposed method.