One of the main challenges of the production of a blue laser is the preparation of defect-free GaN layers. It is of high tehnological interest to characterize GaN nanomembrane mechanically for further advanced applications. The current study addresses the impact of applied stresses on GaN nanomembranes, which are placed on sapphire the substrates, using nanoindentation as a nondestructive test. The mechanical response of the 20, and 100 nm thin GaN nanomembrane were studied at different normal applied loads ranging from 1 mN down to 0.1 mN using the Berkovich nanoindentation technique. There were plastic deformation regions at the nanoindented GaN nanomembranes monitored by the load-displacement (p-h) curves. The depth of the deformed regions increased with increasing the applied loads on the diamond indenter. Beside the insitu depth estimation of the residual nanoindentation using the instrumented nanoindentation machine, Atomic Force Microscopy (AFM) has been deployed as an ex-situ measurements of indentations depth. Scanning Electron Microscopy (SEM) provided us with surface images of the indented membranes. Indentation of the 100 nm thick GaN nanomembrane, where the effect of the substrate is reduced, showed discontinuity in the p-h curves. These discontinuity or pop-in events were attributed to a possible sudden initiation and propagation of threading dislocations in the GaN nanomembrane which was free of threading dislocation upon fabrication. It was suggested to employ µ-Raman spectroscopy methods to investigate the possible structural phase transformation of thicker GaN nanomembranes and to measure the compressive or tensile stresses within the center of the indented zones. Where the observed sudden load-displacements discontinuity or depth excursions during indentation of GaN nanomembranes can be attributed.