We present Keck OSIRIS 1.1-1.8 μm adaptive optics integral field spectroscopy of the planetary-mass companion to GSC 06214-00210, a member of the 5Myr Upper Scorpius OB association. We infer a spectral type of L0 1, and our spectrum exhibits multiple signs of youth. The most notable feature is exceptionally strong Paβ emission (EW = -11.4 0.3), which signals the presence of a circumplanetary accretion disk. The luminosity of GSC06214-00210b combined with its age yields a model-dependent mass of 14 2 M Jup, making it the lowest-mass companion to show evidence of a disk. With a projected separation of 320AU, the formation of GSC06214-00210b and other very low mass companions on similarly wide orbits is unclear. One proposed mechanism is formation at close separations followed by planet-planet scattering to much larger orbits. Since that scenario involves a close encounter with another massive body, which is probably destructive to circumplanetary disks, it is unlikely that GSC06214-00210b underwent a scattering event in the past. This implies that planet-planet scattering is not solely responsible for the population of gas giants on wide orbits. More generally, the identification of disks around young planetary companions on wide orbits offers a novel method to constrain the formation pathway of these objects, which is otherwise notoriously difficult to do for individual systems. We also refine the spectral type of the primary from M1 to K7 and detect a mild (2σ) excess at 22 μm using Wide-Field Infrared Survey Explorer photometry.