The impact of dependence on fossil energy has sparked extensive discussion on a global scale. Ocean wave energy is an alternative solution in the renewable energy transition, considering the abundant availability of energy sources throughout the ocean. Developing wave energy converter (WEC) technology is a strategic step in using sustainable energy. In this research, the effect of gap ratio on the performance of a dual-flap oscillating wave surge converter (OWSC) is investigated comprehensively through a numerical approach. The Boundary Element Method (BEM) describes the interaction between waves and the test device in solving hydrodynamic cases. OWSC dual-flap devices are classified based on three configurations (G/H = 6.9714; 5; 3), tested at varying wave periods. Based on the computational results, the gap ratio significantly influences the performance of the dual-flap OWSC device. The smaller the gap ratio used, the more dominant the shielding effect becomes, especially in the low period, thus affecting the maximum response between the flaps. In addition, the interaction between the two flaps occurs around the gap through radiation force. The computational results show that the optimum gap ratio is found at G/H = 6.9714, with average output power reaching 87.91 Watts on the front-flap and 74.83 Watts on the back-flap. However, the three gap ratio variations are limited to optimum performance in the low wave period so that the mechanical power output decreases as the period increases.