We have reanalyzed archival HST and Subaru data of the recently discovered planetary mass companion (PMC) GQ Lup B. With these we produce the first R- and I-band photometry of the companion and fit a radius and effective temperature using detailed model atmospheres. We find an effective temperature of 2335 ± 100 K, a radius of 0.38 ± 0.05 R, and a luminosity of log(L/L) = -2.42 ± 0.07 (at 140 pc). Since we fit wavelengths that span most of the emitted radiation from GQ Lup, this luminosity estimate is robust, with uncertainty dominated by the distance uncertainty (±50 pc). The radius obtained for 140 pc (0.38 R) is significantly larger than the one originally derived and larger than model predictions. The mass of the object is much more model-dependent than the radiative properties, but for the Gaia dusty models we find a mass between 10M_Jup and 20M_Jup, in the range of the brown dwarf and PMC deuterium-burning boundary. Assuming a distance of 140 pc, observations fit to 1 σ the Baraffe evolution model for an ~15M_Jup brown dwarf. Additionally, the F606W photometric band is significantly overluminous compared to model predictions and other brown dwarfs. Such overluminosity could be explained by a bright Hα emission from chromospheric activity, interaction with another undetected companion, or accretion. Assuming that GQ Lup B has a bright Hα emission line, its Hα emission strength is 10^-1.71±0.10L_bol, significantly larger than field late-type dwarfs. GQ Lup B might be strongly accreting and might still be in its formation phase.