Gd(4)Ge(3-x)Pn(x) (Pn = P, Sb, Bi; x = 0.5-3) phases have been prepared and characterized using X-ray diffraction, wavelength-dispersive spectroscopy, and magnetization measurements. All Gd(4)Ge(3-x)Pn(x) phases adopt a cubic anti-Th3P4 structure, and no deficiency on the Gd or p-element site could be detected. Only one P-containing phase with the Gd4Ge2.51(5)P0.49(5) composition could be obtained, as larger substitution levels did not yield the phase. Existence of Gd4Ge2.51(5)P0.49(5) and Gd4Ge2.49(3)Bi0.51(3) suggests that the hypothetical Gd4Ge3 binary can be easily stabilized by a small increase in the valence electron count and that the size of the p element is not a key factor. Electronic structure calculations reveal that large substitution levels with more electron-rich Sb and Bi are possible for charge-balanced (Gd3+)(4)(Ge4-)(3) as extra electrons occupy the bonding Gd-Gd and Gd-Ge states. This analysis also supports the stability of Gd4Sb3 and Gd4Bi3. All Gd(4)Ge(3-x)Pn(x) phases order ferromagnetically with relatively high Curie temperatures of 234-356 K. The variation in the Curie temperatures of the Gd4Ge3-xSbx and Gd4Ge3-xBix series can be explained through the changes in the numbers of conduction electrons associated with Ge/Sb(Bi) substitution.