  We have measured the $\gamma$-ray energy spectrum from the thermal neutron
capture, ${}^{157}$Gd$(n,\gamma){}^{158}$Gd, on an enriched $^{157}$Gd target
(Gd$_{2}$O$_{3}$) in the energy range from 0.11 MeV up to about 8 MeV. The
target was placed inside the germanium spectrometer of the ANNRI detector at
J-PARC and exposed to a neutron beam from the Japan Spallation Neutron Source
(JSNS). Radioactive sources ($^{60}$Co, $^{137}$Cs, and $^{152}$Eu) and the
reaction $^{35}$Cl($n$,$\gamma$) were used to determine the spectrometer's
detection efficiency for $\gamma$ rays at energies from 0.3 to 8.5 MeV. Using a
Geant4-based Monte Carlo simulation of the detector and based on our data, we
have developed a model to describe the $\gamma$-ray spectrum from the thermal
${}^{157}$Gd($n$,$\gamma$) reaction. While we include the strength information
of 15 prominent peaks above 5 MeV and associated peaks below 1.6 MeV from our
data directly into the model, we rely on the theoretical inputs of nuclear
level density and the photon strength function of ${}^{158}$Gd to describe the
continuum $\gamma$-ray spectrum from the ${}^{157}$Gd($n$,$\gamma$) reaction.
Our model combines these two components. The results of the comparison between
the observed $\gamma$-ray spectra from the reaction and the model are reported
in detail.
