We demonstrate the use of triple axis diffraction measurements, including Φ scans (in which the sample is rotated about an axis perpendicular to its surface) to assess the crystal perfection of wurtzite GaN layers on sapphire grown using different pre-nitridation growth treatments by or-ganometallic vapor phase epitaxy. The Φ scans determine the in-plane misorientation angles between the crystallites and hence provide information on the edge dislocation density. Using glancing incidence (1014) and (1015) reflections, we determined that the misorientation among the GaN crystallites decreases with increasing layer thickness and that the pre-nitridation conditions control the initial level of misorientation. Triple axis ω and ω-2θ scans around the (0002) reflection did not show a systematic trend with increasing layer thickness. However, layers grown without a pre-nitridation step tended to exhibit higher values of both mosaic spread and strain. The appropriate asymmetric reflections for GaN-based Φ scan measurements are determined using structure factor calculations, which are presented here.