  We report a detailed study of the magnetic properties of KBaGd(BO$_3$)$_2$,
in which magnetic Gd$^{3+}$ ($S=7/2$) ions form into two-dimensional triangular
layers. Magnetization, specific heat and magnetocaloric effect (MCE)
measurements have been performed on KBaGd(BO$_3$)$_2$ single crystals. The
results show that a long-range antiferromagnetic state is established below
$T_{\rm N}=0.24$ K. In zero fields, only about half of the full entropy is
released at $T_{\rm N}$, indicating that not all the magnetic moments are
frozen below the ordering temperature, as expected from the geometrical
frustration of the triangular spin lattice. Further studies under external
fields were performed down to 50 mK, and the magnetic phase diagrams are
established with magnetic fields applied both within and perpendicular to the
triangular plane. KBaGd(BO$_3$)$_2$ serves as an example of a two-dimensional
triangular lattice with large spin values ($S=7/2$) and can be directly
compared with the iso-structure KBaR(BO$_3$)$_2$ (R = Dy-Yb) family of doublet
ground states, which exhibit effective spins of $S=1/2$.
