  A search for new phenomena has been performed in final states with at least
one isolated high-momentum photon, jets and missing transverse momentum in
proton--proton collisions at a centre-of-mass energy of $\sqrt{s} = 13$ TeV.
The data, collected by the ATLAS experiment at the CERN LHC, correspond to an
integrated luminosity of 139 $fb^{-1}$. The experimental results are
interpreted in a supersymmetric model in which pair-produced gluinos decay into
neutralinos, which in turn decay into a gravitino, at least one photon, and
jets. No significant deviations from the predictions of the Standard Model are
observed. Upper limits are set on the visible cross section due to physics
beyond the Standard Model, and lower limits are set on the masses of the
gluinos and neutralinos, all at 95% confidence level. Visible cross sections
greater than 0.022 fb are excluded and pair-produced gluinos with masses up to
2200 GeV are excluded for most of the NLSP masses investigated.
