  Graphene plasmons have recently attracted a great deal of attention because
of their tunability, long lifetime, and high degree of field confinement in the
vertical direction. Nearby metal gates have been shown to modify the graphene
plasmon dispersion and further confine their electric field. We study the
plasmons of a graphene sheet deposited on a metal, in the regime in which metal
bands do not hybridize with massless Dirac fermion bands. We derive exact
results for the dispersion and lifetime of the plasmons of such hybrid system,
taking into account metal nonlocalities. The graphene plasmon dispersion is
found to be acoustic and pushed down in energy towards the upper boundary of
the intraband graphene particle-hole continuum, thereby strongly enhancing the
vertical confinement of these excitations. Landau damping of such acoustic
plasmons due to particle-hole excitations in the metal gate is found to be
surprisingly weak, with quality factors exceeding $Q = 10^{2}$.
