  The gadolinium pyrochlores, Gd$_2B_2$O$_7$, are amongst the best realizations
of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We
present a magnetic characterization of Gd$_2$Pt$_2$O$_7$, a unique member of
this family. Magnetic susceptibility, heat capacity, and muon spin relaxation
measurements show that Gd$_2$Pt$_2$O$_7$ undergoes an antiferromagnetic
ordering transition at $T_N = 1.6$ K. This transition is strongly first order,
as indicated by the sharpness of the heat capacity anomaly, thermal hysteresis
in the magnetic susceptibility, and a non-divergent relaxation rate in $\mu$SR.
The form of the heat capacity below $T_N$ suggests that the ground state is an
anisotropic collinear antiferromagnet with an excitation spectrum that is
gapped by 0.245(1) meV. The ordering temperature in Gd$_2$Pt$_2$O$_7$, $T_N =
1.6$ K, is a substantial 160% increase from other gadolinium pyrochlores, which
have been found to order at 1 K or lower. We attribute this enhancement in
$T_N$ to the $B$-site cation, platinum, which, despite being non-magnetic, has
a filled $5d$ $t_{2g}$ orbital and an empty $5d$ $e_g$ orbital that can
facilitate superexchange. Thus, the magnetic frustration in Gd$_2$Pt$_2$O$_7$
is partially "relieved", thereby promoting magnetic order.
