The GERmanium Detector Array (Gerda) experiment located at the INFN Gran Sasso Laboratory (Italy), is looking for the neutrinoless double beta decay of ⁷⁶Ge, by using high-purity germanium detectors made from isotopically enriched material. The combination of the novel experimental design, the careful material selection for radio-purity and the active/passive shielding techniques result in a very low residual background at the Q-value of the decay, about 10^-3 cts/(keV-kg-yr). This makes GERDA the first experiment in the field to be background-free for the complete design exposure of 100 kg-yr. A search for neutrinoless double beta decay was performed with a total exposure of 46.7 kg-yr: 23.2 kg-yr come from the second phase (Phase II) of the experiment, in which the background is reduced by about a factor of ten with respect to the previous phase. The analysis presented in this paper includes 12.4 kg-yr of new Phase II data. No evidence for a possible signal is found: The lower limit for the half-life of ⁷⁶Ge is 8.0 • 10²⁵ yr at 90% CL. The experimental median sensitivity is 5.8 • 10²⁵ yr. The experiment is currently taking data. As it is running in a background-free regime, its sensitivity grows linearly with exposure and it is expected to surpass 10²⁶ yr within 2018.