  The scientific community needs to be prepared to analyse the data from Gaia,
one of the most ambitious ESA space missions, to be launched in 2012. The
purpose of this paper is to provide data and tools in order to predict in
advance how Gaia photometry is expected to be. To do so, we provide
relationships among colours involving Gaia magnitudes and colours from other
commonly used photometric systems (Johnson-Cousins, SDSS, Hipparcos and Tycho).
The most up-to-date information from industrial partners has been used to
define the nominal passbands and based on the BaSeL3.1 stellar spectral energy
distribution library, relationships were obtained for stars with different
reddening values, ranges of temperatures, surface gravities and metallicities.
The transformations involving Gaia and Johnson-Cousins V-I_C and Sloan DSS g-z
colours have the lowest residuals. A polynomial expression for the relation
between the effective temperature and the colour G_BP-G_RP was derived for
stars with T > 4500 K. Transformations involving two Johnson or two Sloan DSS
colours yield lower residuals than using only one colour. We also computed
several ratios of total-to-selective absorption including absorption A_G in the
G band and colour excess E(G_BP-G_RP) for our sample stars. A relationship,
involving A_G/A_V and the intrinsic (V-I_C) colour, is provided. The derived
Gaia passbands have been used to compute tracks and isochrones using the Padova
and BASTI models. Finally, the performances of the predicted Gaia magnitudes
have been estimated according to the magnitude and the celestial coordinates of
the star. The provided dependencies among colours can be used for planning
scientific exploitation of Gaia data, performing simulations of the Gaia-like
sky, planning ground-based complementary observations and for building
catalogues with auxiliary data for the Gaia data processing and validation.
