The Mn5Ge3 exhibits a wide range of fascinating properties such as high spin polarization up to 42 %, large spin diffusion lengths, near-room Curie temperature, and possible integration with Ge, GaAs, and Si substrates. The influence of substrate orientation on magnetic behavior has been investigated. The thin films studied present an in-plane magnetic reversal which is isotropic in samples grown on GaAs(111) and anisotropic in samples grown on GaAs(001). Another difference observed is a linear trend to saturation on the out-of-plane hysteresis loop in samples grown on GaAs(111), comparatively to samples grown on GaAs(001). To better understand this scenario, Density Functional Theory (DFT) calculations were conducted. The results indicate a ferromagnetic metallic ground state with properties that are consistent with the experimental results. The projected densities of states indicate that the main responsible for the magnetic moment observed in the Mn5Ge3 is the Mn-Mn interaction. The magnetic field distribution isosurfaces obtained from the DFT calculations offer a satisfactory explanation of the magnetic reversal experimentally observed, especially the in-plane magnetic behavior in samples grown on both GaAs(111) and GaAs(001) substrates. This work provides a theoretical-experimental approach that allows a deeper understanding of the magnetic anisotropy behavior under the influence of the crystallographic orientation of the substrate observed in nearly-stoichiometric Mn5Ge3 thin films, which is a good candidate for rareearth free magnet in spintronic applications.