  We present 0$.\!\!^{\prime\prime}22$-resolution Atacama Large
Millimeter/submillimeter Array (ALMA) observations of CO(2$-$1) emission from
the circumnuclear gas disk in the red nugget relic galaxy PGC 11179. The disk
shows regular rotation, with projected velocities near the center of 400 km
s$^{-1}$. We assume the CO emission originates from a dynamically cold, thin
disk and fit gas-dynamical models directly to the ALMA data. In addition, we
explore systematic uncertainties by testing the impacts of various model
assumptions on our results. The supermassive black hole (BH) mass
($M_\mathrm{BH}$) is measured to be $M_\mathrm{BH} = (1.91\pm0.04$ [$1\sigma$
statistical] $^{+0.11}_{-0.51}$ [systematic])$\times 10^9$ $M_\odot$, and the
$H$-band stellar mass-to-light ratio $M/L_H=1.620\pm0.004$ [$1\sigma$
statistical] $^{+0.211}_{-0.107}$ [systematic] $M_\odot/L_\odot$. This
$M_\mathrm{BH}$ is consistent with the BH mass$-$stellar velocity dispersion
relation but over-massive compared to the BH mass$-$bulge luminosity relation
by a factor of 3.7. PGC 11179 is part of a sample of local compact early-type
galaxies that are plausible relics of $z\sim2$ red nuggets, and its behavior
relative to the scaling relations echoes that of three relic galaxy BHs
previously measured with stellar dynamics. These over-massive BHs could suggest
BHs gain most of their mass before their host galaxies do. However, our results
could also be explained by greater intrinsic scatter at the high-mass end of
the scaling relations, or by systematic differences in gas- and
stellar-dynamical methods. Additional $M_\mathrm{BH}$ measurements in the
sample, including independent cross-checks between molecular gas- and
stellar-dynamical methods, will advance our understanding of the co-evolution
of BHs and their host galaxies.
