Exciton resonance energies of hexagonal (h-) GaN(0001) epilayers were determined by a combination of high-resolution modulated photoreflectance methods. The results were analyzed thoretically using the Luttinger-Kohn type Hamiltonian for the valence bands under the in-plain biaxial stress, and we obtained the shear deformation potential constants and energy gap in unstrained crystal. Occurrence of the anticrossing of B and C valence bands in tensile biaxially strained h-GaN was suggested.