In the past 15 years, observations of the Galaxy at high energies by Fermi-LAT, AGILE, INTEGRAL and very recently by NuSTAR and eROSITA have been shown to be very exciting, allowing discoveries of a variety of objects and unexpected breakthroughs. However, from a few hundreds of keV to several tens of MeV, the Galaxy remains poorly explored. In this energy range the lack of sufficiently sensitive instruments limits potential discoveries and challenges our understanding of the Galactic high-energy processes and sources. To solve this issue, GECCO is a new mission concept that will allow high-sensitivity observations of the sky from ~50 keV to ~10 MeV. It combines a coded aperture mask technique that provides high angular resolution for source detection, and a Compton telescope that provides high-sensitivity measurements of diffuse emissions. Such a combination enables efficient separation between sources and diffuse emissions. A GECCO-like mission has the potential of answering open questions and leading to new discoveries. Among the most recent challenges regarding the Galaxy, sensitive observations at MeV energies with unprecedented high resolution will open a new window in understanding complicated regions such as the inner Galaxy, the origin of the Fermi Bubbles, the origin of the 511 keV line, and it will provide new insights on element formation in dynamical environments, on possible Galactic winds, and on the mechanisms of propagation of the low-energetic cosmic rays, their sources and their role on the Galaxy evolution.