  We report discovery and characterization of a main-sequence G star orbiting a
dark object with mass $1.90\pm 0.04 M_{\odot}$. The system was discovered via
Gaia astrometry and has an orbital period of 731 days. We obtained multi-epoch
RV follow-up over a period of 639 days, allowing us to refine the Gaia orbital
solution and precisely constrain the masses of both components. The luminous
star is a $\gtrsim 12$ Gyr-old, low-metallicity halo star near the
main-sequence turnoff ($T_{\rm eff}\approx 6000$ K; $\log(g/\left[{\rm
cm\,s^{-2}}\right])\approx 4.0$; $\rm [Fe/H]\approx-1.25$; $M\approx0.79
M_{\odot}$) with a highly enhanced lithium abundance. The RV mass function sets
a minimum companion mass for an edge-on orbit of $M_2 > 1.67 M_{\odot}$, well
above the Chandrasekhar limit. The Gaia inclination constraint, $i=68.7\pm 1.4$
deg, then implies a companion mass of $M_2=1.90\pm0.04 M_{\odot}$. The
companion is most likely a massive neutron star: the only viable alternative is
two massive white dwarfs in a close binary, but this scenario is disfavored on
evolutionary grounds. The system's low eccentricity ($e=0.122\pm 0.002$)
disfavors dynamical formation channels and implies that the neutron star likely
formed with little mass loss ($\lesssim1\,M_{\odot}$) and with a weak natal
kick ($v_{\rm kick}\lesssim 20\,\rm km\,s^{-1}$). The current orbit is too
small to have accommodated the neutron star progenitor as a red supergiant or
super-AGB star. The simplest formation scenario -- isolated binary evolution --
requires the system to have survived unstable mass transfer and common envelope
evolution with a donor-to-accretor mass ratio $>10$. The system, which we call
Gaia NS1, is likely a progenitor of symbiotic X-ray binaries and long-period
millisecond pulsars. Its discovery challenges binary evolution models and bodes
well for Gaia's census of compact objects in wide binaries.
