Transients of fast free-carrier recombination and of multitrapping processes, determined by different types of defects, have been traced by photoluminescence (PL) and contact photoconductivity (CPC) in semi-insulating GaN epitaxial layers. To eliminate effects caused by the electrodes, the CPC decays were supplemented with noninvasive microwave absorption transients. The lifetimes of fast recombination and initial free-carrier capture processes were evaluated using ultraviolet (UV) time-resolved photoluminescence transients. The UV PL band peaked at 3.42 eV with contributions from both stimulated and spontaneous emission was attributed to band-to-band recombination. At the highest excitations, the initial PL decay time exhibited a value of 880 ps due to nonradiative free-carrier recombination. The radiative centers were revealed in continuous-wave PL spectra, where the UV band was accompanied with the bands of blue (B) PL, peaked in the range of 2.82–3.10 eV, and yellow (Y) PL, peaked at 2.19 eV, ascribed to dislocations and bulk donor-acceptor recombination, respectively. The time scale of the relaxation rate exhibited a crossover from picoseconds for stimulated emission to hundreds of nanoseconds for multitrapping. In the asymptotic part, a stretched-exponent decay on the millisecond scale was observed with the disorder factor of α=0.7. The asymptotic decay is explained by competition of centers of nonradiative recombination within bulk of the material and trapping attributed to the dislocations. Behavior of the dislocation-attributed capture centers was simulated using a model of capture cross section, which depends on the excess carrier concentration via screening.