Radiation damping may play an important role in the prediction of gap resonance amplitude by potential flow models. In this paper, the radiation of the gap resonance between twin fixed floating boxes is numerically studied by using a linear potential flow model in frequency domain. An impermeable lid is forced to heave at the free surface in the gap. The characteristics of its added mass and radiation damping, and radiation wave amplitude in the far field are calculated under various geometric conditions. Moreover, the ratio of radiation damping to the viscous damping due to wall friction and/or flow separation is quantified. We find that the ratio of the viscous damping force due to wall friction to radiation damping is approximately 34%~43%, and that due to flow separation to radiation damping is about 140%~320%. The results well explain the prediction performance of potential flow models with/without the viscous damping due to wall friction and/or flow separation.