Long-range magnetic order in the Heisenberg pyrochlore antiferromagnets Gd2Ge2O7 and

$\mathrm{G}{\mathrm{d}}_{2}\mathrm{S}{\mathrm{n}}_{2}{\mathrm{O}}_{7}$ and $\mathrm{G}{\mathrm{d}}_{2}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ have been regarded as good experimental realizations of the classical Heisenberg pyrochlore antiferromagnet with dipolar interaction. The former was found to adopt the Palmer-Chalker state via a single, first-order transition at ${T}_{\mathrm{N}}\ensuremath{\approx}1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, while the latter enters a distinct, partially ordered state through two successive transitions at ${T}_{\mathrm{N}1}\ensuremath{\approx}1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=0.75\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. To shed more light on their distinct magnetic ground states, we have synthesized two more gadolinium-based pyrochlore oxides, $\mathrm{G}{\mathrm{d}}_{2}\mathrm{G}{\mathrm{e}}_{2}{\mathrm{O}}_{7}$ and $\mathrm{G}{\mathrm{d}}_{2}\mathrm{P}{\mathrm{t}}_{2}{\mathrm{O}}_{7}$, under high-pressure conditions and performed detailed characterizations via x-ray powder diffraction, dc and ac magnetic susceptibility, and specific heat measurements down to 100 mK. We found that both compounds enter a long-range antiferromagnetically ordered state through a single, first-order transition at ${T}_{\mathrm{N}}=1.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{G}{\mathrm{d}}_{2}\mathrm{G}{\mathrm{e}}_{2}{\mathrm{O}}_{7}$ and ${T}_{\mathrm{N}}=1.56\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{G}{\mathrm{d}}_{2}\mathrm{P}{\mathrm{t}}_{2}{\mathrm{O}}_{7}$, with the specific heat anomaly similar to that of $\mathrm{G}{\mathrm{d}}_{2}\mathrm{S}{\mathrm{n}}_{2}{\mathrm{O}}_{7}$ rather than $\mathrm{G}{\mathrm{d}}_{2}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$. Interestingly, the low-temperature magnetic specific heat values of both $\mathrm{G}{\mathrm{d}}_{2}\mathrm{G}{\mathrm{e}}_{2}{\mathrm{O}}_{7}$ and $\mathrm{G}{\mathrm{d}}_{2}\mathrm{P}{\mathrm{t}}_{2}{\mathrm{O}}_{7}$ were found to follow nicely the ${T}^{3}$ dependence as expected for a three-dimensional antiferromagnet with gapless spin-wave excitations. We have rationalized the enhancement of ${T}_{\mathrm{N}}$ in terms of the reduced Gd-Gd distances for the chemically pressurized $\mathrm{G}{\mathrm{d}}_{2}\mathrm{G}{\mathrm{e}}_{2}{\mathrm{O}}_{7}$ and the addition of extra superexchange pathways through the empty $\mathrm{Pt}\ensuremath{-}{e}_{g}$ orbitals for $\mathrm{G}{\mathrm{d}}_{2}\mathrm{P}{\mathrm{t}}_{2}{\mathrm{O}}_{7}$. Our current study has expanded the family of gadolinium-based pyrochlores and permits us to achieve a better understanding of their distinct magnetic properties in a more comprehensive perspective.