This study employs a multiscale simulation method coupling the two-temperature model with molecular dynamics to systematically investigate the microscopic mechanisms underlying the swift heavy ion (SHI) velocity effect on irradiation damage in gallium nitride (GaN). The simulations describe the coupled energy transfer between the electronic and lattice subsystems on the femtosecond timescale, together with transient lattice melting, recrystallization, and microscopic defect evolution on the atomic scale. The results indicate that, under similar electronic energy loss conditions, a lower incident ion velocity leads to highly localized energy deposition and consequently more severe lattice damage. Taking Ta ion irradiation as an example, when the incident velocity decreases from 6.47 MeV/u to 2.71 MeV/u, the continuous ion tracks composed of N₂ bubbles become more pronounced. Specifically, the radius of the amorphous core expands from 2.21 nm to 2.46 nm, and the maximum diameter of the central N₂ bubbles enlarges from 3.2 nm to 3.6 nm. Meanwhile, N₂ molecules linear density rises from 8.25×10⁸ cm^-1 to 11.37×10⁸ cm^-1, indicating that low-velocity ions induce more severe decomposition of the wurtzite GaN lattice. Furthermore, there is a proliferation in the densities of both the abundant Ga-rich and N-deficient metastable zincblende GaN nanodomains generated around the ion tracks and the screw dislocation networks that exhibit a strong spatial correlation with them. The high density of microscopic defects induced by low-velocity ions could elevate the probability of forming vertical leakage channels, thus serving as a critical potential factor that exacerbates the risk of single-event burnout in GaN devices. This study provides an essential theoretical basis and data support for understanding the electrical performance degradation mechanisms of GaN devices under SHI irradiation and for advancing radiation-hardening designs.