Résumé : The scope of this paper is twofold. First, it describes the simulation scenario and the results of the large scale double-blind test campaign set-up by the Planetary System Working Group for the realistic simulation of the Gaia capabilities in detecting extra-solar planets. Present limitations and envisaged future improvements are also discussed. Then, the identified capabilities are put in context by highlighting the unique contributions that the Gaia exo-planet discoveries will be able to bring to the science of extra-solar systems of the next decade.