The role of Si-doping in enhancing the magnetic ordering temperature (T_c) of Gd₅(Si_xGe_1–x )₄ giant magnetocaloric compounds was investigated using x-ray magnetic circular dichroism (XMCD) and diamond anvil cell (DAC) techniques. The purpose of the study is to understand the mechanism of doping-induced ferromagnetic order in these compounds that may advance the magnetic refrigeration technology. The results demonstrate that hydrostatic pressure leads to similar effects as Si-doping for x ≥ 0.125 because the P-T phase diagram reproduces the most notable features of the x-T phase diagram, indicating that the magnetic properties of these compounds are volume-driven. The low-x (0 < x ≤ 0.75) region exhibits an inhomogeneous magneto-structural ground state featured by a mixed antiferromagnetic (orthorhombic (II))–ferromagnetic (orthorhombic (I)) phase at low temperature. Pressure was found to remove this magneto-structural inhomogeneity by fully restoring the magnetization that is obtained for x ≥ 0.125. However, unlike the nearly constant dT_c/dP obtained for 0.125 ≤ x < 0.5, dT_c/dP of the low-x samples is strongly x-dependent. This suggests that the emergence of the ferromagnetic order from within the antiferromagnetic phase of Gd ₅Ge₄ parent compound cannot be simply described as a volume-effect due to the existence of the magneto-structural inhomogeneity. Finally, the quantitative correspondence between Si-doping and hydrostatic pressure was examined in order to know if the properties of these materials are monotonically volume-dependent. It was found that Si-doping increases T_c much more effectively than pressure, by a factor of ∼ 11 for a given volume reduction. A local lattice contraction was found around Si atoms as a result of the substitution of Ge by the smaller Si atoms resulting in a remarkably high local chemical pressure. This local contraction results in a stronger Si 3p-Gd 5d orbital hybridization benefiting the indirect ferromagnetic exchange and hence responsible for a more effective T_c increase, overthrowing the concept prior to this study that macroscopic volume contraction is the major course determining T_c increase.