Electron capture behaviours for major trips in thin epitaxial and thick free-standing GaN samples have been experimentally and theoretically studied by using deep-level transient spectroscopy (DLTS). According to the logarithmic dependence of the DLTS signal on the filling pulse width, most of the traps in thin epitaxial GaN layers with high dislocation density behave as line defects. In sharp contrast, the same traps in thick free-standing GaN layers with low dislocation density behave as point defects. The most likely explanation for these phenomena is that the electron traps in question tend to segregate around dislocations, when present in large numbers.