  Context. On September 2016 the first data from Gaia were released (DR1). The
first release included photometry for over 10^9 sources in the very broad G
system. Aims. To test the correspondence between G magnitudes in DR1 and the
synthetic equivalents derived using spectral energy distributions from observed
and model spectrophotometry. To correct the G passband curve and to measure the
zero point in the Vega system. Methods. I have computed the synthetic G and
Tycho-2 BV photometry for a sample of stars using the Next Generation Spectral
Library (NGSL) and the Hubble Space Telescope (HST) CALSPEC spectroscopic
standards. Results. I have found that the nominal G passband curve is too blue
for the DR1 photometry, as shown by the presence of a color term in the
comparison between observed and synthetic magnitudes. A correction to the
passband applying a power law in lambda with an exponent of 0.783 eliminates
the color term. The corrected passband has a Vega zero point of 0.070$\pm$0.004
magnitudes.
