The $\gamma$-ray spectrum produced from the thermal neutron capture on enriched gadolinium targets ($^{155}$Gd and $^{157}$Gd) were measured using the ANNRI Germanium Spectrometer at J-PARC. We have also built a $\gamma$-ray emission model of $^{158}$Gd decay, in which the $\gamma$-rays were classified into the discrete prompt $\gamma$ rays and continuum spectrum. We compared the data and our model, and found a fair agreement between them at the level of 26% over the $\gamma$-ray energy from 0.2 MeV to 8.0 MeV. In this proceedings, the measured spectra and the model are described.