  Herein, we report the discovery of a novel Gd-based magnetic compound
GdPt$_2$B with a chiral crystal structure. X-ray diffraction and chemical
composition analyses reveal a CePt$_2$B-type crystal structure (space group:
$P6_422$) for GdPt$_2$B. Moreover, we successfully grew single crystals of
GdPt$_2$B using the Czochralski method. Magnetization measurements and the
Curie$-$Weiss analysis demonstrate that the ferromagnetic interaction is
dominant in GdPt$_2$B. A clear transition is observed in the temperature
dependence of electrical resistivity, magnetic susceptibility, and specific
heat at $T_{\rm O}$ = 87 K. The magnetic phase diagram of GdPt$_2$B, which
consists of a field-polarized ferromagnetic region and a magnetically ordered
region, resembles those of known chiral helimagnets. Furthermore, magnetic
susceptibility measurements reveal a possible spin reorientation within the
magnetically ordered phase in magnetic fields perpendicular to the screw axis.
The results demonstrate that GdPt$_2$B is a suitable platform for investigating
the competing effects of ferromagnetic and antisymmetric exchange interactions
in rare-earth-based chiral compounds.
