X-ray single crystal and powder diffraction studies on the Gd₅Ga_xGe_4-x system with 0 ≤ x ≤ 2.2 reveal dependence of interslab T−T dimer distances and crystal structures themselves on valence electron concentration (T is a mixture of Ga and Ge atoms). While the Gd₅Ga_xGe_4-x phases with 0 ≤ x ≤ 0.6 and valence electron concentration of 30.4−31 e^-/formula crystallize with the Sm₅Ge₄-type structure, in which all interslab T−T dimers are broken (distances exceeding 3.4 Å), the phases with 1 ≤ x ≤ 2.2 and valence electron concentration of 28.8−30 e-/formula adopt the Pu₅Rh₄- or Gd₅Si₄-type structures with T−T dimers between the slabs. An orthorhombic Pu₅Rh₄-type structure, which is intermediate between the Gd₅Si₄- and Sm₅Ge₄-type structures, has been identified for the Gd₅GaGe₃ composition. Tight-binding linear-muffin-tin-orbital calculations show that substitution of three-valent Ga by four-valent Ge leads to larger population of the antibonding states within the dimers and, thus, to dimer stretching and eventually to dimer cleavage.