This paper proposes a method for synthesis of hexagonal GaN on Si(100) and Si(113) substrates, where nanostructures with an element size less than 100 nm are formed on the surface. It has been established that the method of gas-phase epitaxy from metalorganic compounds in a hydrogen atmosphere on such substrates makes it possible to form semipolar layers of GaN(10-11) and GaN(11-22) with a minimum half-width of the X-ray diffraction swing curve of about 30 arcmin. It is shown that during the formation of a semipolar AlN layer at the initial stage of epitaxy, a corrugated surface is formed on NP-Si(100) from the semipolar planes AlN(10-11) and AlN(10-1-1) with counter-directional c axes. Then, during the growth of the GaN layer, a transition is made from the symmetrical state of the semipolar GaN(10-11) and GaN(10-1-1) planes to an asymmetric state with the orientation of the c axis of the GaN(10-11) layer. That transition is apparently determined by the difference in the values of the surface energy of GaN during epitaxy on the corrugated surface.