Gaia is the ESA cornerstone mission set to revolutionise our understanding of the Milky Way. This proposal will shape a critical mass of new expertise with the fundamental skills required to power the scientific exploitation of Gaia over the coming decade and beyond.The GREAT-ITN research theme is 'Unravelling the Milky Way' focused on four fundamental problems: unravelling the origin and history of our home galaxy; tracing the birth place and understanding the astrophysical properties of the stellar constituents of our galaxy; deepening the understanding of planetary systems by linking the study of exoplanets to the origins of the solar system; take up the grand challenges offered by Gaia in the domains of the distance scale and the transient sky.The GREAT-ITN will deliver a training programme structured around these research themes to a core of new researchers, equipping them with the skills and expertise to become future leaders in astronomy or enter industry. These skills are relevant across many of the key challenges facing us now from climate change to energy security. These require well trained people, people which this GREAT-ITN will deliver.The 12 GREAT-ITN partners in Europe, and 1 in China, each have world leading expertise. 19 additional associate partners provide access to complementary expertise and facilities. The network includes three associates from the Information Technology industry: Microsoft, InterSystems and The Server Labs, each driving the new global 'on-line' agenda. The European Space Agency provides the vital interface to the Gaia project, and exposure to the Space industry.This powerful combination of expertise, from industry and academia, will lead to a new cluster of expertise in the area of Galactic astronomy, deliver powerful and effective training to a large pool of Early Stage Researchers, and cement a sustainable research community adding impact to Europe's leadership in this fundamental area of astrophysics.