A model of basal plane stacking faults as boundaries between incoherently scattering domains in m-plane GaN films is reviewed. m-Plane GaN films are analyzed with a modified version of the Williamson–Hall analysis in order to determine the length-scale of coherent scattering and tilt-mosaic contribution to X-ray rocking curve widths for the primary in-plane directions. This analysis shows that basal plane stacking faults are the predominant source of rocking-curve width anisotropy in the m-plane films, and indicate that the modified Williamson–Hall analysis can be used as a non-destructive technique for measuring basal plane stacking fault densities in m-GaN films.