Two or more structures arranged side by side with narrow gaps may be suffered from large-amplitude free-surface oscillations, which could cause green water on the deck and lead to dramatic increase of hydrodynamic loading acting on structures. Here, transient resonant motions of the free surface inside a narrow gap between two fixed boxes triggered by focused transient wave groups with various focused wave amplitudes are simulated using a two-dimensional numerical wave tank. The free-surface amplifications not only inside the gap but in the vicinity of the two-box system, the response time and the damping time of the transient gap resonance, the maximum wave loads on both boxes and the relative importance of the higher-order wave loads to the first-order ones are systematically investigated. It is found that the most vulnerable position to green water closely depends on the incident focused wave amplitude. The damping time decreases gradually with increasing focused wave amplitude, while the response time seems insensitive to the latter. As the focused wave amplitude increases, the normalized maximum wave loads on both boxes are also shown to decline gradually overall, while the relative importance of the higher-order wave loads to the first-order ones becomes more and more remarkable.