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The study reports the influence of rare-earth ion doping on the structural, magnetic, and magnetocaloric properties of ferrimagnetic Gd_3−<i>x</i>RE<i>_x</i>Fe₅O₁₂ (RE = Y, Nd, Sm, and Dy, <i>x</i> = 0.0, 0.25, 0.50, and 0.75) garnet compound prepared via facile autocombustion method followed by annealing in air. X-Ray diffraction (XRD) data analysis confirmed the presence of a single-phase garnet. The compound’s lattice parameters and cell volume varied according to differences in ionic radii of the doped rare-earth ions. The RE³⁺ substitution changed the site-to-site bond lengths and bond angles, affecting the magnetic interaction between site ions. Magnetization measurements for all RE³⁺-doped samples demonstrated paramagnetic behavior at room temperature and soft-ferrimagnetic behavior at 5 K. The isothermal magnetic entropy changes (−Δ<i>S_M</i>) were derived from the magnetic isotherm curves, <i>M</i> vs. <i>T</i>, in a field up to 3 T in the Gd_3−<i>x</i>RE<i>_x</i>Fe₅O₁₂ sample. The maximum magnetic entropy change (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>−</mo><mo>∆</mo><msubsup><mrow><mi>S</mi></mrow><mrow><mi>M</mi></mrow><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msubsup></mrow></semantics></math></inline-formula>) increased with Dy³⁺ and Sm³⁺substitution and decreased for Nd³⁺ and Y³⁺ substitution with <i>x</i> content. The Dy³⁺-doped Gd_2.25Dy_0.75Fe₅O₁₂ sample showed <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>−</mo><mo>∆</mo><msubsup><mrow><mi>S</mi></mrow><mrow><mi>M</mi></mrow><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msubsup></mrow></semantics></math></inline-formula>~2.03 Jkg⁻¹K⁻¹, which is ~7% higher than that of Gd₃Fe₅O₁₂ (1.91 Jkg⁻¹K⁻¹). A first-principal density function theory (DFT) technique was used to shed light on observed properties. The study shows that the magnetic moments of the doped rare-earths ions play a vital role in tuning the magnetocaloric properties of the garnet compound.