The anisotropic effective electron masses in wurtzite GaN are determined by generalized infrared spectroscopic ellipsometry. Nonpolar (11-20) oriented thin films allow accessing both effective masses, m∗⊥ and m∗∥ , by determining the screened plasma frequencies. A n-type doping range up to 1.7E20 cm−3 is investigated. The effective mass ratio m∗⊥/m∗∥ is obtained with highest accuracy and is found to be 1.11 independent on electron concentration up to 1.2E20 cm−3. For higher electron concentrations, the conduction band non-parabolicity is mirrored in changes. Absolute values for effective electron masses depend on additional input of carrier concentrations determined by Hall effect measurements. We obtain m∗⊥=(0.239±0.004)m0 and m∗∥=(0.216±0.003)m0 for the parabolic range of the GaN conduction band. Our data are indication of a parabolic GaN conduction band up to an energy of approximately 400 meV above the conduction band minimum.