Gd3+ ESR at high fields in Gd-doped high-T-c superconductors is shown to be a precise
probe of the conduction electron static spin susceptibility, chi. The Gd3+ ESR Knight
shift at 245 GHz and the spin relaxation broadening at 245 and 9.2 GHz in Gd0.01Y0.99Ba2Cu3Oy
(with various values of between 6.05 and 7.0) are in agreement with published Y-89
NMR data above 70 K and provide additional measurements at the rare earth site at
much lower temperatures. In the superconducting compounds chi(T) differs strongly
from the Yoshida function and at low T it follows a T or T-2 dependence. An anomalous
ESR line shape is found at low T in the superconducting state; we provisionally attribute
this to a small (10(-2)-10(-3) mu(B)) static magnetic moment on planar Cu sites.
