Experimental and numerical studies are performed to investigate drillship moonpool gap resonance in both stationary and transit conditions in a wave flume. This study contains an assessment of the influence of size and depth of the moonpool on the gap resonance phenomenon. An openfoam-based computational fluid dynamics (CFD) model was established, and the numerical data show good agreement with measurements from the model tests. Both piston and sloshing mode gap resonances are clearly observed. This study shows that the gap resonance frequency and wave elevation response amplitude operator (RAO) inside the moonpool are dependent on its dimensions, and the transit speed of the drillship and wave direction significantly influences the characteristics of gap resonances. It is noticed that the nearness of the wave flume sidewalls significantly influences the piston and sloshing wave elevation RAO at certain frequencies regardless of moonpool length and draft.