A spatio-time-resolved cathodoluminescence (STRCL) system was constructed by replacing the electron beam (e-beam) gun of a conventional scanning electron microscope by the in-house manufactured pulsed e-beam gun, which is excited using femtosecond laser pulses. By using this system, STRCL measurements were carried out on low threading dislocation density freestanding GaN substrates grown by hydride vapor phase epitaxy. High-resolution cathodoluminescence imaging allows for visualization of nonradiative recombination channels in the vicinity of accidentally formed domain boundaries. Local cathodoluminescence lifetimes (τCL) for the near-band-edge (NBE) emission are shown to be sensitively position dependent. A linear relation between the equivalent internal quantum efficiency (ηint eq) and τCL for the NBE emission was observed at room temperature under a weak excitation condition, and spatially resolved excitation led to the observation of the highest ηint eq of 20 % with τCL of 3.3 ns.