ESA's cornerstone mission Gaia is scheduled to be launched in late 2011. It will determine the positions and distances of more than 24 million stars within 3 kpc of the Sun with a 1% relative accuracy. About 150,000 O and B-type stars are expected in the whole sample, as well as additional peculiar stars such as Be, B[e], and WR stars. Gaia will also provide spectrophotometry over the spectral range 330-1150 nm (RP/BP bands) and medium-resolution spectroscopy in the domain ranging from 847 to 874 nm (RVS spectral range). A large effort is now under way to develop the necessary software aiming at automatically derive the astrophysical parameters and radial velocities. One component of this development is the computation of grids of synthetic spectra for different types of stars and, in particular, for massive stars.