Gliese86 is a nearby K-dwarf hosting a giant planet on a ~16day orbit and an outer white dwarf companion on a ~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 Gl86 system to constrain its primordial orbit when both stars were on the main sequence; the closest approach between the two stars was then about 9au. Such a close separation limited the size of the protoplanetary disk of Gl86A and dynamically hindered the formation of the giant planet around it. Our measurements of Gl86B and Gl86Ab's orbits reveal Gl86 as a system in which giant planet formation took place in a disk truncated at ~2au. Such a disk would be just big enough to harbor the dust mass and total mass needed to assemble Gl86Ab's core and envelope, assuming a high disk accretion rate and a low viscosity. Inefficient accretion of the disk onto Gl86Ab, however, would require a disk massive enough to approach the Toomre stability limit at its outer truncation radius. The orbital architecture of the Gl86 system shows that giant planets can form even in severely truncated disks and provides an important benchmark for planet formation theory.