  The spin dynamics in the geometrically frustrated pyrochlore antiferromagnet
$\rm Gd_2Sn_2O_7$ is studied by means of the electron spin resonance. In the
ordered phase ($T_N = 1$ K), we have detected three gapped resonance modes.
Their values agree well with the developed spin-wave theory which takes into
account the Heisenberg nearest-neighbor exchange, the single-ion anisotropy and
the long-range dipolar interactions. The theory also predicts a fourth
lowest-frequency gap, which lies beyond the experimental range of frequencies,
but determines the exponential decrease of the specific heat at low
temperature.
