  The intermediate-mass star Beta Pictoris is known to be surrounded by a
structured edge-on debris disk within which a gas giant planet was discovered
orbiting at 8-10 AU. The physical properties of Beta Pic b were previously
inferred from broad and narrow-band 0.9-4.8 microns photometry. We used
commissioning data of the Gemini Planet Imager (GPI) to obtain new astrometry
and a low-resolution (R=35-39) J-band (1.12-1.35 microns) spectrum of the
planet. We find that the planet has passed the quadrature. We constrain its
semi-major axis to $\leq$ 10 AU (90 % prob.) with a peak at 8.9+0.4-0.6 AU. The
joint fit of the planet astrometry and the most recent radial velocity
measurements of the star yields a planet's dynamical mass $\leq$ 20 MJup
(greater than 96 % prob.). The extracted spectrum of Beta Pic b is similar to
those of young L1-1.5+1 dwarfs. We use the spectral type estimate to revise the
planet luminosity to log(L/Lsun)=-3.90+-0.07. The 0.9-4.8 microns photometry
and spectrum are reproduced for Teff=1650+-150 K and a log g lower than 4.7 dex
by 12 grids of PHOENIX-based and LESIA atmospheric models. If we adopt the most
recent system age estimate (21+-4 Myr), the bolometric luminosity and the
constraints on the dynamical mass of Beta Pic b are only reproduced by warm-
and hot-start tracks with initial entropies Si greater than 10.5 kB/baryon.
Such initial conditions may result from an inefficient accretion shock and/or a
planetesimal density at formation higher than in the classical core accretion
model. Considering a younger age for the system or a conservative formation
time for Beta Pic b does not change these conclusions.
