  The relationship between luminosity and mass is of fundamental importance for
direct imaging studies of brown dwarf and planetary companions to stars. In
principle this can be inferred from theoretical mass-luminosity models;
however, these relations have not yet been thoroughly calibrated, since there
is a lack of substellar companions for which both the brightness and mass have
been directly measured. One notable exception is GJ 758 B, a brown dwarf
companion in a $\sim$20 AU orbit around a nearby Sun-like star, which has been
both directly imaged and dynamically detected through a radial velocity trend
in the primary. This has enabled a mass constraint for GJ 758 B of
42$^{+19}_{-7}$ $M_{\rm Jup}$. Here, we note that Gaia is ideally suited for
further constraining the mass of intermediate-separation companions such as GJ
758 B. A study of the differential proper motion, $\Delta \mu$, with regards to
HIPPARCOS is particularly useful in this context, as it provides a long time
baseline for orbital curvature to occur. By exploiting already determined
orbital parameters, we show that the dynamical mass can be further constrained
to $42.4^{+5.6}_{-5.0}\, M_{\rm Jup}$ through the Gaia-HIPPARCOS $\Delta \mu$
motion. We compare the new dynamical mass estimate with substellar evolutionary
models and confirm previous indications that there is significant tension
between the isochronal ages of the star and companion, with a preferred stellar
age of $\leq 5$ Gyr while the companion is only consistent with very old ages
of $\geq 8$ Gyr.
