  Stellar mass is a fundamental parameter that is key to our understanding of
stellar formation and evolution, as well as the characterization of nearby
exoplanet companions. Historically, stellar masses have been derived from
long-term observations of visual or spectroscopic binary star systems. While
advances in high-resolution imaging have enabled observations of systems with
shorter orbital periods, stellar mass measurements remain challenging, and
relatively few have been precisely measured. We present a new statistical
approach to measuring masses for populations of stars. Using Gaia astrometry,
we analyze the relative orbital motion of $>3,800$ wide binary systems
comprising low-mass stars to establish a Mass-Magnitude relation in the Gaia
$G_\mathrm{RP}$ band spanning the absolute magnitude range
$14.5>M_{G_\mathrm{RP}}>4.0$, corresponding to a mass range of
$0.08$~M$_{\odot}\lesssim M\lesssim1.0$~M$_{\odot}$. This relation is directly
applicable to $>30$ million stars in the Gaia catalog. Based on comparison to
existing Mass-Magnitude relations calibrated for 2MASS $K_{s}$ magnitudes, we
estimate that the internal precision of our mass estimates is $\sim$10$\%$. We
use this relation to estimate masses for a volume-limited sample of
$\sim$18,200 stars within 50~pc of the Sun and the present-day field mass
function for stars with $M\lesssim 1.0$~M$_{\odot}$, which we find peaks at
0.16~M$_{\odot}$. We investigate a volume-limited sample of wide binary systems
with early K dwarf primaries, complete for binary mass ratios $q>0.2$, and
measure the distribution of $q$ at separations $>100$~au. We find that our
distribution of $q$ is not uniformly distributed, rather decreasing towards
$q=1.0$.
