In this work the transformation kinetics of GaN pseudomorphic layer and the lattice constant evolution of 2D GaN “frozen” layer under sequential switching off/on of ammonia flow at a growth temperature of 740 °C were investigated  by reflection high energy electron diffraction method (RHEED). It was shown by the Bragg spot kinetics intensity of GaN layer that when ammonia flow is turned off, the intensity of Bragg spot reaches saturation and does not change during the exposure time in vacuum, while the maximum achieved intensity decreases when ammonia flow is turned off/on sequentially. Hence there is practically no effect of thermal decomposition on the change in the morphology of the GaN layer. It was found experimentally that the GaN layer formed with each cycle of 2D "frozen" is partially relaxed, which is explained within the Mariette equilibrium model. Thus, relaxation of elastic energy of 2D "frozen" GaN  layer is due to the fact that some amount of 3D islands remains on the surface and the decrease of elastic energy value is caused by losses for islands faceting maintenance.