  Gamma-ray spectra from cosmic-ray proton and electron interactions with dense
gas clouds have been calculated using a Monte Carlo event simulation code,
GEANT4. Such clouds are postulated as a possible form of baryonic dark matter
in the Universe. The simulation fully tracks the cascade and transport
processes that are important in a dense medium, and the resulting gamma-ray
spectra are computed as a function of cloud column-density. These calculations
are used for predicting the Galactic diffuse gamma-ray spectrum that may be
contributed by baryonic dark matter; the results are compared with data from
the EGRET instrument, and used to constrain the fraction of Galactic dark
matter that may be in the form of dense gas clouds. In agreement with previous
authors, we find useful constraints on the fraction of Galactic dark matter
that may be in the form of low column-density clouds ($\Sigma \la 10 g
cm^{-2}$). However, this fraction rises steeply in the region $\Sigma \sim 10^2
g cm^{-2}$, and for $\Sigma \ga 200 g cm^{-2}$ we find that baryonic dark
matter models are virtually unconstrained by the existing gamma-ray data.
