  The possibility to observe the nuclear modification of the gluon distribution
at small-x (gluon shadowing) using high-p_T prompt photon production at RHIC
and at LHC is discussed. The per-nucleon ratio, sigma(p+A -> gamma+X) / A
sigma(p+p -> gamma+X), is computed for both inclusive and isolated prompt
photons in perturbative QCD at NLO using different parametrizations of nuclear
parton densities, in order to assess the visibility of the shadowing signal.
The production of isolated photons turns out to be a promising channel which
allows for a reliable extraction of the gluon density, G^A/G^p, and the
structure function, F_2^A/F_2^p, in a nucleus over that in a proton. Moreover,
the production ratio of prompt photons at forward-over-backward rapidity in p-A
collisions provides an estimate of G^A/G^p (at small x) over F_2^A/F_2^p (at
large x), without the need of p-p reference data at the same energy.
