The cathodoluminescence spectrum of n-GaN crystal has been studied with emphasis placed on its self-absorption behavior. An improved analysis of cathodoluminescence spectral shift with electron penetration depth is shown, according to the following approach: (i) a main band and a sub-band, both of Gaussian nature, are used for spectral convolution; (ii) the observed absorption edge is still described in terms of the Urbach rule, but the Urbach tail is experimentally retrieved by directly using the absorption spectrum (at low energies) obtained from standard cathodoluminescence spectra; (iii) an experimental approach is used to retrieve the actual morphology of the cathodoluminescence probe.