Dislocation-related luminescence (DRL) in n-GaN from a-screw dislocations introduced by cracking or nanoindentation is investigated using cathodoluminescence (CL) and polarization optical spectroscopy. In strain-free samples with rare array of parallel dislocations, the polarization direction one of two DRL spectral components is directed parallel while for other components, it is inclined under the angle of about 40° to the dislocation line that is attributed to the theoretically predicted different orientations of chemical bonds in the cores of N and Ga partials of the dissociated a-screw dislocation. DRL investigations near the indentation prick reveal the presence of two doublet broadbands polarized at 60° with respect to each other that is caused by two crossing families of screw dislocations observed with CL.