  In this first paper we simulate the population of disc Red Clump stars to be
observed by Gaia. We generate a set of test particles and we evolve it in a 3D
barred Milky Way like galactic potential. We assign physical properties of the
Red Clump trace population and a realistic 3D interstellar extinction model. We
add Gaia observational constraints and an error model according to the
pre-commissioning scientific performance assessments. We present and analyse
two mock catalogues, offered to the community, that are an excellent test bed
for testing tools being developed for the future scientific exploitation of
Gaia data. The first catalogue contains stars up to Gaia G 20, while the second
is the subset containing Gaia radial velocity data with a maximum error of
sigmaVr=10 kms. Here we present first attempts to characterise the density
structure of the Galactic bar in the Gaia space of observables. The Gaia large
errors in parallax and the high interstellar extinction in the inner parts of
the Galactic disc prevent us to model the bar overdensity. This result suggests
the need to combine Gaia and IR data to undertake such studies. We find that IR
photometric distances for this Gaia sample allow us to recover the Galactic bar
orientation angle with an accuracy of approximately 5 degrees.
