We study photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Mg-doped n-GaN grown by hydride vapour-phase epitaxy. Defect-related red and blue emission bands around 1.85 eV and 2.90 eV, respectively, are observed in addition to the yellow emission band. The red and the blue emission bands are attributed to the recombinations involving the same Mg-related defects according to the result of the thermal annealing experiment. The PLE spectra of the red, the yellow, and the blue emission bands show that all of these emission bands can be interpreted in terms of a configuration coordinate (CC) diagram. The blue emission at 2.90 eV is attributed to the transition from the conduction band to the Mg-related deep acceptors. The CC diagram shows that the yellow luminescence is due to the transition from a deep donor state to a shallow acceptor state. A possible origin of the deep donor level is also discussed.