Although the Milky Way bulge is our closest opportunity to study in detail such a complex chemo-dynamical system, its formation and evolution is still poorly understood. The high extinction, the crowding, and the superposition of multiple structures along the line of sight make studies of the inner Galactic regions challenging. Two main scenarios have been invoked for the bulge formation: gravitational collapse or hierarchical merging of subclumps and secular evolution of the disc through a bar forming a pseudo-bulge. Recent results suggest that the Galactic bulge could be the result of both formation processes. Gaia will be limited by the extinction and the crowding but will still observe accurately more than 20 million bulge stars (Robin et al. 2005). The Gaia parallaxes and proper motions will probe the structure of the Galactic inner regions, will allow to obtain a clean bulge sample and will solve the different biases in distance currently present in the bulge surveys. Radial velocities will also be obtained for the brightest stars in the less crowded regions, providing 6D constrains to dynamical models. Completed by ground-based abundances, the Gaia bulge survey will provide unique constraints on the formation history of the Galactic inner regions.