Magnetization and low field ac susceptibility measurements have been made on Gd_<70>Y_<30> single crystal alloys. Low isofield susceptibility data for the c-axis sample exhibit a sharp peak at the highest ordering temperature , which suggests the existence of the antiphase domain structure in the c-axis ferromagnet region with basal-plane disorder. The antiphase domain structure is easily supressed by the applied field of about 20 Oe in the susceptibility measurement.