  Frustrated lanthanide oxides with dense magnetic lattices are of fundamental
interest for their potential in cryogenic refrigeration due to a large ground
state entropy and suppressed ordering temperatures, but can often be limited by
short-range correlations. Here, we present examples of frustrated fcc oxides,
Ba$_2$GdSbO$_6$ and Sr$_2$GdSbO$_6$ and the new site-disordered analog
Ca$_2$GdSbO$_6$ ([CaGd]$_A$[CaSb]$_B$O$_6$), in which the magnetocaloric effect
is influenced by minimal superexchange ($J_1 \sim 10$ mK). We report on the
crystal structures using powder x-ray diffraction and the bulk magnetic
properties through low-field susceptibility and isothermal magnetization
measurements. The Gd compounds exhibit a magnetic entropy change of up to -15.8
J/K/mol$_\textrm{Gd}$ in a field of 7 T at 2 K, a 20 % excess compared to the
value of -13.0 J/K/mol$_\textrm{Gd}$ for a standard in magnetic refrigeration,
Gd$_3$Ga$_5$O$_{12}$. Heat capacity measurements indicate a lack of magnetic
ordering down to 0.4 K for Ba$_2$GdSbO$_6$ and Sr$_2$GdSbO$_6$, suggesting
cooling down through the liquid 4-He regime. A mean-field model is used to
elucidate the role of primarily free spin behavior in the magnetocaloric
performance of these compounds in comparison to other top-performing Gd-based
oxides. The chemical flexibility of the double perovskites raises the
possibility of further enhancement of the magnetocaloric effect in the
Gd$^{3+}$ fcc lattices.
