  We report a tunable spin mixing conductance, up to $\pm 22\%$, in a
Y${}_{3}$Fe${}_{5}$O${}_{12}$/Platinum (YIG/Pt) bilayer.This control is
achieved by applying a gate voltage with an ionic gate technique, which
exhibits a gate-dependent ferromagnetic resonance line width. Furthermore, we
observed a gate-dependent spin pumping and spin Hall angle in the Pt layer,
which is also tunable up to $\pm$ 13.6\%. This work experimentally demonstrates
spin current control through spin pumping and a gate voltage in a YIG/Pt
bilayer, demonstrating the crucial role of the interfacial charge density for
the spin transport properties in magnetic insulator/heavy metal bilayers.
