The primary goal of the GERmanium Detector Array (GERDA) experiment at the Laboratori Nazionali del Gran Sasso of INFN is the search for the neutrinoless double beta decay of 76Ge. High-purity germanium detectors made from material enriched in 76Ge are operated directly immersed in liquid argon, allowing for a substantial reduction of the background with respect to predecessor experiments. The first 5.04 kg yr of data collected in Phase I of the experiment have been analyzed to measure the half-life of the neutrino-accompanied double beta decay of 76Ge. The observed spectrum in the energy range between 600 and 1800 keV is dominated by the double beta decay of 76Ge. The half-life extracted from GERDA data is T2ν1/2 = (1.84+0.14−0.10) × 1021 yr.