  The GERmanium Detector Array (GERDA) collaboration searched for neutrinoless
double-$\beta$ decay in $^{76}$Ge with an array of about 40 high-purity
isotopically-enriched germanium detectors. The experimental signature of the
decay is a monoenergetic signal at Q$_{\beta\beta}$ = 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-$\beta$ decays allowed by the Standard Model. The
energy resolution and stability were determined and monitored as a function of
time using data from regular $^{228}$Th 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.
