  We study the effects of the Gauss-Bonnet term on the energy spectrum of
inflationary gravitational waves. The models of inflation are classified into
two types based on their predictions for the tensor power spectrum: red-tilted
($n_T<0$) and blue-tilted spectra ($n_T>0$), respectively, and then the energy
spectra of the gravitational waves are calculated for each type of model. We
find that the gravitational wave spectra are enhanced depending on the model
parameter if the predicted inflationary tensor spectra have a blue tilt,
whereas they are suppressed for the spectra that have a red tilt. Moreover, we
perform the analyses on the reheating parameters involving the temperature, the
equation-of-state parameter, and the number of $e$-folds using the
gravitational wave spectrum. Our results imply that the Gauss-Bonnet term plays
an important role not only during inflation but also during reheating whether
the process is instantaneous or lasts for a certain number of $e$-folds until
it thermalizes and eventually completes.
