Magnetic resonance of Cu and of Gd in insulating GdSr$_{2}$Cu$_{2}$NbO$_{8}$ and in superconducting GdSr$_{2}$Cu$_{2}$RuO$_{8}$

Weak-ferromagnetic (or anti-ferromagnetic) resonance of Cu and electron spin resonance (ESR) of Gd are observed both in insulating GdSr$_{2}$Cu$_{2}$NbO$_{8}$ and in superconducting GdSr$_{2}$Cu$_{2}$RuO$_{8}$. The Cu resonance implies that the CuO$_2$ planes are magnetic and indicates that the superconducting layer of GdSr$_2$Cu$_2$RuO$_8$ is SrO (not CuO$_2$), as in its related superconducting compound without cuprate-planes, doped Sr$_2$YRuO$_6$.