Since 2012, zinc gallogermanate has drawn interests as an excellent host phosphor for a variety of dopants (e.g., Cr3+, Bi3+ and Mn2+). However, the origin of its self-activated luminescence has been largely unknown. Here, zinc gallogermanate of the composition Zn1+xGa2-2xGexO4 (0x1) is prepared by solid-state reaction, and the evolution of the crystal structure with the composition is studied. The phosphors show a broad white-bluish emission upon excitation by ultraviolet (UV) light, and the luminescence intensity greatly increases as Ga3+ are substituted by Ge4+. A full spectrum of the defects and trap centers responsible for the luminescence is given by a multiple characterization methods, such as low temperature electron spin resonance, positron annihilation lifetime and thermoluminescence spectra. The results show that the zinc gallogermanate is a promising self-activated phosphor for a variety of applications.