  The $\mu\nu$SSM solves the $\mu$ problem of supersymmetric models and
reproduces neutrino data, simply using couplings with right-handed neutrinos
$\nu$'s. Given that these couplings break explicitly $R$ parity, the gravitino
is a natural candidate for decaying dark matter in the $\mu \nu$SSM. In this
work we carry out a complete analysis of the detection of $\mu \nu$SSM
gravitino dark matter through $\gamma$-ray observations. In addition to the
two-body decay producing a sharp line, we include in the analysis the
three-body decays producing a smooth spectral signature. We perform first a
deep exploration of the low-energy parameter space of the $\mu \nu$SSM taking
into account that neutrino data must be reproduced. Then, we compare the
$\gamma$-ray fluxes predicted by the model with Fermi-LAT observations. In
particular, with the 95$\%$ CL upper limits on the total diffuse extragalactic
$\gamma$-ray background using 50 months of data, together with the upper limits
on line emission from an updated analysis using 69.9 months of data. For
standard values of bino and wino masses, gravitinos with masses larger than
about 4 GeV, or lifetimes smaller than $10^{28}$ s, produce too large fluxes
and are excluded as dark matter candidates. However, when limiting scenarios
with large and close values of the gaugino masses are considered, the
constraints turn out to be less stringent, excluding masses larger than 17 GeV
and lifetimes smaller than $4\times 10^{25}$ s.
