Straight segments of a-screw dislocations introduced by scratching of basal (0001) of intentionally undoped low-ohmic GaN radiate a doublet of narrow luminescent lines in the spectral region at about 3.1-3.2 eV while the dislocation intersection points possess luminescence band at about 3.3 eV. Transmission electron microscopy reveals that the dislocation cores are dissociated into two 30⁰ partials separated by stacking fault (SF) ribbon with the width of 4-6 nm width and that the dislocation nodes contain extended SF of sizes of 25-30 nm. Dislocation-related luminescence (DRL) is ascribed to exciton bound by the states of partial dislocation cores and of SF quantum well. The increase of the SF lateral sizes is assumed to cause the DRL spectral shift between straight dislocations and their nodes due to the system dimensionality transition from 1D to 2D respectively.