  Despite the availability of a spin Hamiltonian for the Gd3Ga5O12 garnet (GGG)
for over twenty five years, there has so far been little theoretical insight
regarding the many unusual low temperature properties of GGG. Here we
investigate GGG in zero magnetic field using mean-field theory. We reproduce
the spin liquid-like correlations and, most importantly, explain the positions
of the sharp peaks seen in powder neutron diffraction experiments. We show that
it is crucial to treat accurately the long-range nature of the magnetic dipolar
interactions to allow for a determination of the small exchange energy scales
involved in the selection of the experimental ordering wave vector. Our results
show that the incommensurate order in GGG is classical in nature, intrinsic to
the microscopic spin Hamiltonian and not caused by weak disorder.
