  We report a polarised neutron study of the magnetic structures and phase
trasitions in \gdc\ in low magnetic fields. These experiments have been
complemented by integrated intensity measurements with unpolarised neutrons in
zero field. Polarised 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 behaviour of the
coherent magnetic scattering from the Cu sublattice, which shows sharp
intensity minima at \Tcn1 $\approx 18$K and \Tcn2 $\approx 8$ K, is due to
cross-overs in the sign of the interaction between strongly coupled, weakly
ferromagnetic, CuO$_2$ layers. At \Tcn1\ the coupling changes from
ferromagnetic to anti-ferromagnetic and long-range order between layers is
temporarily lost. \Tcn2\ is the temperature at which the Gd moments order and a
further reorganisation 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 Gd$_2$CuO$_4$.
