GAMMA-400 is a new space mission, designed as a dual experiment, capable to study both high energy gamma rays (from $sim$100 MeV to few TeV) and cosmic rays (electrons up to 20 TeV and nuclei up to $sim$10$^{15}$ eV). The full simulation framework of GAMMA-400 is based on the Geant4 toolkit. The details of the gamma-ray reconstruction pipeline in the pre-shower and calorimeter will be outlined. The performance of GAMMA-400 (PSF, effective area) have been obtained using this framework. The most updated results on them will be shown.