  The availability of reliable bolometric corrections and reddening estimates,
rather than the quality of parallaxes will be one of the main limiting factors
in determining the luminosities of a large fraction of Gaia stars. With this
goal in mind, we provide Gaia G, BP and RP synthetic photometry for the entire
MARCS grid, and test the performance of our synthetic colours and bolometric
corrections against space-borne absolute spectrophotometry. We find indication
of a magnitude-dependent offset in Gaia DR2 G magnitudes, which must be taken
into account in high accuracy investigations. Our interpolation routines are
easily used to derive bolometric corrections at desired stellar parameters, and
to explore the dependence of Gaia photometry on Teff, log(g), [Fe/H],
alpha-enhancement and E(B-V). Gaia colours for the Sun and Vega, and
Teff-dependent extinction coefficients, are also provided.
