The full-energy-peak efficiency of HPGe detectors at γ-ray energies around 10 MeV is not easily accessible with experimental methods. Monte-Carlo simulations with Geant4 can provide these efficiencies. G4Horus is a ready-to-use Geant4 application for the HORUS HPGe-detector array. Users can configure the modular parts to match their experiment with minimal knowledge of the simulation software and limited time commitment. In our case, knowing and implementing the geometry with high precision is the biggest challenge. To implement the different target chambers, we transform the existing CAD models to Geant4 geometry with CADMesh. We also found a large discrepancy between experimental and simulated efficiency for some older HPGe detectors, which could be remedied by introducing a large dead region around the inner core. This project is open source and available from https://github.com/janmayer/G4Horus [1]. We invite everyone to adapt the project or adopt parts of the code for other projects.