Observations with high vertical resolution have revealed that power spectra of horizontal wind and temperature fluctuations versus vertical wavenumber m have a universal shape with a steep slope in a high m range, approximately proportional to m^(-3). Several theoretical models explaining this spectral slope were proposed under an assumption of gravity wave (GW) saturation. However, little evidence has been obtained to show that these universal spectra are fully composed of GWs. To confirm the validity of this assumption, two kinds of m spectra are calculated using outputs from a GW-permitting high-top general circulation model. One is the spectra for GWs designated by fluctuations having total horizontal wavenumbers of 21–639. The other is the spectra of fluctuations unfiltered except extracting a linear trend in the vertical that were often analyzed in the observational studies. Comparison between the two shows that GWs dominate the observed spectra only in a higher m part of the steep slope, whereas disturbances other than GWs significantly contribute to its lower m part. Moreover, geographical distributions of the characteristic wavenumbers, slopes, and spectral densities of GW spectra are examined for several divided height regions of the whole middle atmosphere. It is shown that strong vertical shear below the zonal wind jets as well as the wave saturation is responsible for the formation of the steep slopes of GW spectra.