  The antiferromagnetically (AFM) ordered state of GdRh$_{2}$Si$_{2}$ which
consists of AFM-stacked ferromagnetic layers is investigated by magnetic
resonance spectroscopy. The almost isotropic Gd$^{3+}$ paramagnetic resonance
becomes anisotropic in the AFM ordered region below 107 K. The emerging
internal anisotropic exchange-fields are still small enough to allow an
investigation of their magnetization dynamics by using a standard
microwave-frequency magnetic resonance technique. We could characterize this
anisotropy in detail in the ferromagnetic layers of the excitation at 9 and 34
GHz. We derived a resonance condition for the AFM ordered phase to describe the
weak in-plane anisotropic behaviour in combination with a mean-field analysis.
