  Aims. The Radial Velocity Spectrometer (RVS) on board the ESA satellite
mission Gaia has no calibration device. Therefore, the radial velocity zero
point needs to be calibrated with stars that are proved to be stable at a level
of 300 m/s during the Gaia observations. Methods. We compiled a dataset of
~71000 radial velocity measurements from five high-resolution spectrographs. A
catalogue of 4813 stars was built by combining these individual measurements.
The zero point was established using asteroids. Results. The resulting
catalogue has seven observations per star on average on a typical time baseline
of six years, with a median standard deviation of 15 m/s. A subset of the most
stable stars fulfilling the RVS requirements was used to establish the radial
velocity zero point provided in Gaia Data Release 2. The stars that were not
used for calibration are used to validate the RVS data.
