The present research focuses on the experimental and numerical analysis of nonlinear gap resonance which can occur for two side-by-side configured vessels during the offloading operation of Liquified Natural Gas (LNG). The proximity of the FLNG (Floating Liquefied Natural Gas) facility and LNGC (Liquified Natural Gas Carrier) brings about the formation of a long narrow gap region between the two vessels. The model test was carried out in a wave flume with vessels of different sizes to obtain the resonance response at different locations in the gap region with the effects of gap distances and vessel drafts in incident waves of different wave frequencies and wave directions analysed. It was found that certain model configurations produce increased wave amplification between the vessels, these were highlighted and presented and have the possibility of being predicted to prevent the occurrence. The numerical analysis was carried out with the potential flow solver SIMA in the time domain and a calibrated damping factor was assigned to suppress the overestimated wave elevation in the narrow gap region.