  We report Gemini Planet Imager H band high-contrast imaging/integral field
spectroscopy and polarimetry of the HD 100546, a 10 $Myr$-old early-type star
recently confirmed to host a thermal infrared bright (super)jovian protoplanet
at wide separation, HD 100546 b. We resolve the inner disk cavity in polarized
light, recover the thermal-infrared (IR) bright arm, and identify one
additional spiral arm. We easily recover HD 100546 b and show that much of its
emission originates an unresolved, point source. HD 100546 b likely has
extremely red infrared colors compared to field brown dwarfs, qualitatively
similar to young cloudy superjovian planets, however, these colors may instead
indicate that HD 100546 b is still accreting material from a circumplanetary
disk. Additionally, we identify a second point source-like peak at $r_{proj}$
$\sim$ 14 AU, located just interior to or at inner disk wall consistent with
being a 10--20 $M_{J}$ candidate second protoplanet-- "HD 100546 c" -- and
lying within a weakly polarized region of the disk but along an extension of
the thermal IR bright spiral arm. Alternatively, it is equally plausible that
this feature is a weakly polarized but locally bright region of the inner disk
wall. Astrometric monitoring of this feature over the next 2 years and emission
line measurements could confirm its status as a protoplanet, rotating disk hot
spot that is possibly a signpost of a protoplanet, or a stationary emission
source from within the disk.
