We report a polarized neutron study of the magnetic structures and phase transitions in Gd2CuO4 in low magnetic fields. These experiments have been complemented by integrated intensity measurements with unpolarized neutrons in zero field. Polarized neutron flipping ratio measurements have been made with magnetic fields H=0.05, 0.10, and 0.5 T in the temperature range 4-20 K. These have enabled us to deduce that the anomalous temperature behavior of the coherent magnetic scattering from the Cu sublattice, which shows sharp intensity minima at TC1 ≈18 K and TC2 ≈8 K, is due to cross overs in the sign of the interaction between strongly coupled, weakly ferromagnetic, CuO2 layers. At TC1 the coupling changes from ferromagnetic to antiferromagnetic and long-range order between layers is temporarily lost. TC2 is the temperature at which the Gd moments order and a further reorganization of the interlayer order takes place. The weak ferromagnetism of the CuO layers is found to be due to a small rotation of the Cu moments in the same direction as that in which their coordinating oxygen squares rotate in the tetragonal to orthorhombic distortion of the crystal structure. Further analysis of the flipping ratio measurements has enabled us to model the magnetic structures of the zero-field and the field-induced phases of Gd2CuO4.