  We present high spatial resolution (FWHM$\sim$0.14'') observations of the
CO($8-7$) line in GDS-14876, a compact star-forming galaxy at $z=2.3$ with
total stellar mass of $\log(M_{\star}/M_{\odot})=10.9$. The spatially resolved
velocity map of the inner $r\lesssim1$~kpc reveals a continous velocity
gradient consistent with the kinematics of a rotating disk with $v_{\rm
rot}(r=1\rm kpc)=163\pm5$ km s$^{-1}$ and $v_{\rm rot}/\sigma\sim2.5$. The
gas-to-stellar ratios estimated from CO($8-7$) and the dust continuum emission
span a broad range, $f^{\rm CO}_{\rm gas}=M_{\rm gas}/M_{\star}=13-45\%$ and
$f^{\rm cont}_{\rm gas}=50-67\%$, but are nonetheless consistent given the
uncertainties in the conversion factors. The dynamical modeling yields a
dynamical mass of$\log(M_{\rm dyn}/M_{\odot})=10.58^{+0.5}_{-0.2}$ which is
lower, but still consistent with the baryonic mass, $\log$(M$_{\rm bar}$=
M$_{\star}$ + M$^{\rm CO}_{\rm gas}$/M$_{\odot}$)$=11.0$, if the smallest
CO-based gas fraction is assumed. Despite a low, overall gas fraction, the
small physical extent of the dense, star-forming gas probed by CO($8-7$),
$\sim3\times$ smaller than the stellar size, implies a strong concentration
that increases the gas fraction up to $f^{\rm CO, 1\rm kpc}_{\rm gas}\sim 85\%$
in the central 1 kpc. Such a gas-rich center, coupled with a high
star-formation rate, SFR$\sim$ 500 M$_{\odot}$ yr$^{-1}$, suggests that
GDS-14876 is quickly assembling a dense stellar component (bulge) in a strong
nuclear starburst. Assuming its gas reservoir is depleted without
replenishment, GDS-14876 will quickly ($t_{\rm depl}\sim27$ Myr) become a
compact quiescent galaxy that could retain some fraction of the observed
rotational support.
