  Sound velocity and attenuation measurements on the frustrated garnet material
Gd3Ga5O12 (GGG) are presented as a function of field and temperature, using two
different acoustic modes and with two different magnetic field orientations:
[100] and [110]. We demonstrate that the phase diagram is highly anisotropic,
with two distinct field-induced ordered phases for H||[110] and only one for
H||[100]. Extensive lattice softening is found to occur at low fields, which
can be associated with spin fluctuations. However, deep within the spin liquid
phase a low-temperature stiffening of the lattice and reduced attenuation
provides evidence for a spin gap which may be related to short-range
antiferromagnetic correlations over minimal 10-spin loops.
