  R$_2$Mo$_2$O$_7$ compounds show a ferromagnetic metal-insulator spin glass
transition tuned by the radius of the rare earth ion R$^{3+}$. We have studied
Gd$_2$Mo$_2$O$_7$ located on the verge of the transition, by neutron
diffraction on a $^{160}$Gd isotopic sample, $\mu$SR and X ray diffraction
using the synchrotron radiation. All measurements were done both at ambient and
under applied pressure. At ambient pressure, a ferromagnetic state is observed
below the Curie temperature (T$_{\rm C}$ = 70 K). The ordered magnetic moments
at 1.7 K are parallel and equal to 5.7(5) $\mu_{\rm B}$ and 0.8(2) $\mu_{\rm
B}$ for Gd and Mo, respectively. The relaxation rate measured by $\mu$SR
evidences strong spin fluctuations below T$_{\rm C}$ and down to the lowest
temperature (6.6 K). A spin reorientation occurs in the range 20
K$<$T$<$T$_{\rm C}$. The ferromagnetic state is strongly unstable under
pressure. T$_{\rm C}$ sharply decreases (down to 38 K at 1.3 GPa) and Bragg
peaks start to coexist with mesoscopic ferromagnetic correlations. The ordered
moments decrease under pressure. At 2.7 GPa long range magnetic order
completely breaks down. In this spin glass state, Gd-Gd spin correlations
remain ferromagnetic with a correlation length limited to the fourth neighbor,
and Gd-Mo spin correlations turn to antiferromagnetic. The unique combination
of three microscopic probes under pressure provides a detailed description of
the magnetic transition, crucial for further theories.
