  We combine Keck/HIRES radial velocities, imaging with HiCIAO/Subaru and the
Hubble Space Telescope, and absolute astrometry from Hipparcos and Gaia to
measure a dynamical mass of $70 \pm 5$ Jupiter masses for the brown dwarf
companion to Gl 229. Gl 229B was the first imaged brown dwarf to show clear
signs of methane in its atmosphere. Cooling models have been used to estimate a
mass in the range of 20 - 55 Jupiter masses, much lower than our measured
value. We argue that our high dynamical mass is unlikely to be due to
perturbations from additional unseen companions or to Gl 229B being itself a
binary, and we find no evidence of a previously claimed radial velocity planet
around Gl 229A. Future Gaia data releases will confirm the reliability of the
absolute astrometry, though the data pass all quality checks in both Hipparcos
and Gaia. Our dynamical mass implies a very old age for Gl 229, in some tension
with kinematic and activity age indicators, and/or shortcomings in brown dwarf
cooling models. Gl 229B joins a small but growing list of T dwarfs with masses
approaching the minimum mass for core hydrogen ignition.
