  Using the Gemini Planet Imager (GPI) located at Gemini South, we measured the
near-infrared (1.0-2.4 micron) spectrum of the planetary companion to the
nearby, young star $\beta$ Pictoris. We compare the spectrum obtained with
currently published model grids and with known substellar objects and present
the best matching models as well as the best matching observed objects.
Comparing the empirical measurement of the bolometric luminosity to
evolutionary models, we find a mass of $12.9\pm0.2$ $\mathcal{M}_\mathrm{Jup}$,
an effective temperature of $1724\pm15$ K, a radius of $1.46\pm0.01$
$\mathcal{R}_\mathrm{Jup}$, and a surface gravity of $\log g = 4.18\pm0.01$
[dex] (cgs). The stated uncertainties are statistical errors only, and do not
incorporate any uncertainty on the evolutionary models. Using atmospheric
models, we find an effective temperature of $1700-1800$ K and a surface gravity
of $\log g = 3.5$-$4.0$ [dex] depending upon model. These values agree well
with other publications and with "hot-start" predictions from planetary
evolution models. Further, we find that the spectrum of $\beta$ Pic b best
matches a low-surface gravity L2$\pm$1 brown dwarf. Finally comparing the
spectrum to field brown dwarfs we find the the spectrum best matches 2MASS
J04062677-381210 and 2MASS J03552337+1133437.
