Structural and magnetic properties of the new Bi-doped Gd2−xBixCuO4 system are reported. With the doping of larger trivalent Bi3+ ions, the T′ phase solubility region is retained in the range of 0⩽x⩽0.1. Weak ferromagnetic or canted antiferromagnetic ordering temperature TN(Cu) of Cu moments decreases from 282 K for Gd2CuO4 to 258 K for Gd1.93Bi0.07CuO4 and 256 K for Gd1.9Bi0.1CuO4. Due to small internal field Hint associated with weak ferromagnetic order, the spin reorientation temperature Tsr(Cu) is field dependent and decreases from 24 K in 1 G to 12 K in 1 kG for Gd1.95Bi0.05CuO4. Preliminary powder x-ray Rietveld refinement analysis using the T′ phase ideal oxygen position indicates a large temperature factor B for the oxygen sites in the Cu–O plane and suggests that the degree of oxygen structural distortion may be closely related to the strength of weak ferromagnetic order.