  We present intermediate resolution spectroscopy of the optical counterpart to
the black hole X-ray transient MAXI J1820+070 (=ASASSN-18ey) obtained with the
OSIRIS spectrograph on the 10.4-m Gran Telescopio Canarias. The observations
were performed with the source close to the quiescent state and before the
onset of renewed activity in August 2019. We make use of these data and K-type
dwarf templates taken with the same instrumental configuration to measure the
projected rotational velocity of the donor star. We find $v_{rot} \sin i = 84
\pm 5$ km s$^{-1}$ ($1\!-\!\sigma$), which implies a donor to black-hole mass
ratio $q = {M_2}/{M_1} = 0.072 \pm 0.012$ for the case of a tidally locked and
Roche-lobe filling donor star. The derived dynamical masses for the stellar
components are $M_1 = (5.95 \pm 0.22)\sin ^{-3}i$ $M_\odot$ and $M_2 = (0.43
\pm 0.08) \sin^{-3}i$ $M_\odot$. The use of $q$, combined with estimates of the
accretion disk size at the time of the optical spectroscopy, allows us to
revise our previous orbital inclination constraints to $66^{\circ} < i <
81^{\circ}$. These values lead to 95% confidence level limits on the masses of
$5.73 <M_1(M_\odot) < 8.34$ and $0.28 < M_2(M_\odot) < 0.77$. Adopting instead
the $63 \pm 3^{\circ}$ orientation angle of the radio jet as the binary
inclination leads to $M_1 = 8.48^{+0.79}_{-0.72} M_\odot$ and $M_2 =
0.61^{+0.13}_{-0.12} M_\odot$ ($1\!-\!\sigma$).
