Three novel Zintl phase solid solutions in the Ca11−xGdxSb10−y(0.36(2) ≤x≤ 0.58(4), 0.26(1) ≤y≤ 0.37(1)) system have been successfully synthesized by arc melting, and their crystal structures were carefully characterized by powder and single-crystal X-ray diffraction analyses. During the synthesis, as we changed the reaction time, we were able to observe the early stages of the crystal growth process for a cubic-shaped single crystal with hummocky-type patterns on it. All three title compounds adopted the tetragonalI4/mmm(Z= 4, Pearson codetI84) space group and crystallized in the Ho11Ge10-type phase. The overall complex crystal structure contained three Ca/Gd mixed sites with particular site preferences and one Sb site with some occupational deficiencies. A series of DFT calculations and detailed structure analyses proved that the observed cationic site preference should be attributed to the size-factor criterion rather than the electronic-factor criterion, and the Gd-containing title compounds became a tipping point for the size factor criterion in the overall Ca11−xRExSb10−y(RE = rare-earth metals) series. Theoretical investigations indicated that the energetically unfavorable antibonding character of the Sb-Sb interaction on the Sb4tetramers causedca.2.6-3.7% (fractional percentage) of Sb deficiencies and eventually resulted in keeping the p-type character of the Ca11−xGdxSb10−ysystem despite the successful n-type Gd doping. According to the temperature-dependent thermoelectric property measurements, the relatively “heavier” Gd dopants successfully improved the electrical conductivity, Seebeck coefficient, and thermal conductivity of Ca10.63(4)Gd0.37Sb9.74(1)compared to those of two previously reported Ca10.75(3)Nd0.26Sb9.82(1)and Ca10.82(4)Sm0.18Sb9.86(1), which contained the relatively “lighter” Nd and Sm dopants.