The resonant water motion inside a narrow gap between two identical fixed boxes that are in side-by-side configuration is investigated using a two-dimensional (2D) numerical wave tank based on OpenFOAM ® , an open source CFD package. Gap resonance is excited by regular waves with various wave heights, ranging from linear waves to strong nonlinear waves. This paper mainly focuses on the harmonic analyses of the free-surface elevation in the narrow gap and wave loads (including the horizontal wave forces, the vertical wave forces and the moments) on the bodies. It is found that the influences of the incident wave height on the higher-order harmonic components of different physical quantities are quite different. The effects of the incident wave height on the reflection, transmission and energy loss coefficients are also discussed. Finally, aiming at the quantitative estimation of the response time and the damping time of gap resonance, two different methods are proposed and verified for the first time on gap resonance.