The correlation between magnetic and structural transitions in Gd5SixGe4−x hampers the studies of valence electron concentration (VEC) effects on magnetism. Such studies require decoupling of the VEC-driven changes in the magnetic behavior and crystal structure. The designed compounds, Gd5GaSb3 and Gd5GaBi3, adopt the same Sm5Ge4-type structure as Gd5Ge4 while the VEC increases from 31  e−/formula in Gd5Ge4 to 33  e−/formula in Gd5GaPn3 (Pn: pnictide atoms). As a result, the antiferromagnetic ground state in Gd5Ge4 is tuned into the ferromagnetic one in Gd5GaPn3. First-principles calculations reveal that the nature of interslab magnetic interactions is changed by introducing extra p electrons into the conduction band, forming a ferromagnetic bridge between the adjacent [∝2Gd5T4] slabs.