Heteroepitaxial GaN thin films are deposited on (0001) sapphire substrates using ion-beam-assisted deposition with different content of hexagonal (h-GaN) and cubic (c-GaN) polytypes. X-ray diffraction is used to characterize crystallographic orientation, stresses and unstressed lattice parameters separately for each polytype. The typical compressive stress values for h- and c-GaN are about 195 and 155 MPa, respectively. The stress state in h-GaN is not influenced by the presence of c-GaN and, within one layer, stresses formed in c-GaN are always smaller then the stresses in h-GaN. A typical unstressed lattice parameter a for c-GaN is 0.45035(1) nm and, for h-GaN, unstressed lattice parameters a and c are 0.31881(2) and 0.51831(1) nm, respectively. In the thin film deposited with high ion energy of 50 eV, two regions with different structural properties are detected. The region close to the interface exhibits significantly higher stresses and also larger unstressed lattice parameters in comparison to the surface part of the thin film which is partly stress-relaxed.