The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double-β decay in ⁷⁶Ge with an array of about 40 high-purity isotopically-enriched germanium detectors. The experimental signature of the decay is a monoenergetic signal at Q_ββ= 2039.061 (7) keV in the measured summed energy spectrum of the two emitted electrons. Both the energy reconstruction and resolution of the germanium detectors are crucial to separate a potential signal from various backgrounds, such as neutrino-accompanied double-β decays allowed by the Standard Model. The energy resolution and stability were determined and monitored as a function of time using data from regular ²²⁸Th calibrations. In this work, we describe the calibration process and associated data analysis of the full Gerda dataset, tailored to preserve the excellent resolution of the individual germanium detectors when combining data over several years.