  Gliese 86 is a nearby K dwarf hosting a giant planet on a $\approx$16-day
orbit and an outer white dwarf companion on a $\approx$century-long orbit. In
this study we combine radial velocity data (including new measurements spanning
more than a decade) with high angular resolution imaging and absolute
astrometry from Hipparcos and Gaia to measure the current orbits and masses of
both companions. We then simulate the evolution of the Gl 86 system to
constrain its primordial orbit when both stars were on the main sequence; the
closest approach between the two stars was then about $9\,$AU. Such a close
separation limited the size of the protoplanetary disk of Gl 86 A and
dynamically hindered the formation of the giant planet around it. Our
measurements of Gl 86 B and Gl 86 Ab's orbits reveal Gl 86 as a system in which
giant planet formation took place in a disk truncated at $\approx$2$\,$AU. Such
a disk would be just big enough to harbor the dust mass and total mass needed
to assemble Gl 86 Ab's core and envelope, assuming a high disk accretion rate
and a low viscosity. Inefficient accretion of the disk onto Gl 86 Ab, however,
would require a disk massive enough to approach the Toomre stability limit at
its outer truncation radius. The orbital architecture of the Gl 86 system shows
that giant planets can form even in severely truncated disks and provides an
important benchmark for planet formation theory.
