With the next Gaia release (expected in April 2018), the distance of about 300 Galactic Cepheids will be derived with a precision of better than 3%. These distances will be used first to constrain the Cepheid period-luminosity relation, but they will also bring strong constrains on the physics of Cepheids, through the projection factor, a physical quantity used in the inverse Baade-Wesselink (BW) method.