  Low frequency transport measurements are performed on GdSr$_2$RuCu$_2$O$_8$
pellets. The observed current-voltage curves are qualitatively explained in the
framework of a simple phenomenological model accounting for coexistence of
ferromagnetism and superconductivity in the sample. A Curie temperature
$T_{cM}$=133 K and a superconducting critical temperature $T_{cS}$=18 K, with
an onset temperature $T_{cO}$=33 K, are extracted from the analysis of the
current-voltage curves.
