The temperature dependencies of the g-factors in GdBa₂Cu₃O_6+x crystal are successfully explained using the suggestion that the big macroscopic magnetization of the samples due to the high concentration of Gd³⁺ ions (100 %) is responsible for the ESR signal. The difference of the temperature behavior of g-factors on the 35 GHz and 9.3 GHz is due to a different fraction of the penetration depth of the field in the superconductor.