  The GERmanium Detector Array (GERDA) experiment searched for the
lepton-number-violating neutrinoless double-$\beta$ ($0\nu\beta\beta$) decay of
$^{76}$Ge, whose discovery would have far-reaching implications in cosmology
and particle physics. By operating bare germanium diodes, enriched in
$^{76}$Ge, in an active liquid argon shield, GERDA achieved an unprecedently
low background index of $5.2\times10^{-4}$ counts/(keV$\cdot$kg$\cdot$yr) in
the signal region and met the design goal to collect an exposure of 100
kg$\cdot$yr in a background-free regime. When combined with the result of Phase
I, no signal is observed after 127.2 kg$\cdot$yr of total exposure. A limit on
the half-life of $0\nu\beta\beta$ decay in $^{76}$Ge is set at
$T_{1/2}>1.8\times10^{26}$ yr at 90% C.L., which coincides with the sensitivity
assuming no signal.
