  We present the measurement of the two-neutrino double-$\beta$ decay rate of
$^{76}$Ge performed with the GERDA Phase II experiment. With a subset of the
entire GERDA exposure, 11.8 kg$\cdot$yr, the half-life of the process has been
determined: $T^{2\nu}_{1/2} = (2.022 \pm 0.018_{stat} \pm
0.038_{sys})\times10^{21}$ yr. This is the most precise determination of the
$^{76}$Ge two-neutrino double-$\beta$ decay half-life and one of the most
precise measurements of a double-$\beta$ decay process. The relevant nuclear
matrix element can be extracted: $M^{2\nu}_{\text{eff}} = (0.101\pm0.001).$
