Observations with high vertical resolution have revealed that power spectra of horizontal wind and temperature fluctuations versus the vertical wavenumber m have common shape with a steep slope of ~m^-3. Several theoretical models explaining this spectral slope were proposed on the basis of an assumption of gravity wave (GW) saturation. However, there is little to confirm that such observed spectra are fully composed of GWs. To examine whether the m^-3 spectra are attributable to GWs, spectral analysis of GWs in the middle atmosphere was conducted using a GW-permitting high-top general circulation model. The model-simulated spectra have steep spectral slopes consistent with observations. GWs were extracted as fluctuations having total horizontal wavenumbers of 21–639. Comparison of the spectra of GWs with those of all simulated fluctuations revealed that GWs dominate only when m is high, whereas disturbances other than GWs largely contribute to the spectra when m is low, even in the m^-3 range. Additionally, vertical and geographical distributions of the characteristic wavenumbers, slopes, and amplitudes of GW spectra were also examined. It is inferred that strong vertical shear below the eastward and westward jets is responsible for the formation of the steep spectral slopes of GWs.