We have measured the infrared reflectance anisotropy of bulk wurzite (0001) GaN. Reflectance difference (RD) spectra have been determined from the experimental reflectance for light polarization along the sample axes [01¯10] and [112¯0]. The spectra show an optical anisotropy around the frequency of the E1(LO) and A1(LO) GaN phonons. The spectra are influenced by the structural quality of the samples, as revealed by measurements of high resolution x-ray diffraction and Raman scattering. The RD may be originated by inhomogeneous anisotropic broadening of the optical phonons induced by crystal size distribution. Based on the sensitivity of the infrared spectra to the crystalline quality, we propose this infrared technique as a useful optical probe for hexagonal GaN characterization.