  We report the magneto-optical response of Gadolinium Gallium Garnet (GGG) and
Gadolinium Scandium Gallium Garnet (SGGG) at frequencies ranging from $300 \,
\mathrm{GHz}$ to $1 \, \mathrm{THz}$, and determine the material response
tensor. Within this frequency window, the materials exhibit nondispersive and
low-loss optical responses. At low temperatures, significant THz Faraday
rotations are found in the (S)GGG samples. Such strong gyroelectric response is
likely associated with the high-spin paramagnetic state of the Gd$^{3+}$ ions.
A model of the material response tensor is determined, together with the Verdet
and magneto-optic constants.
