<p>The ferrite samples Mn_0.5Mg_0.5-xCu_xGd_0.2Fe_1.8O₄ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) were prepared via the solid-state route and exhibit multiphase characteristics due to partial substitution of Cu²⁺ and Gd³⁺. Simultaneous substitution of Cu²⁺ into the Mn/Mg A/B sites while keeping a fixed Gd³⁺ addition is relatively uncommon. Gd³⁺ (large ionic radius, 4f electrons) introduces new magnetic/structural effects not present in simple binary ferrites. FESEM and AFM analyses show increased porosity and surface roughness with Cu content. UV-Vis spectroscopy demonstrated a reduction in the optical forbidden band gap from 2.38 to 1.68&#xa0;eV with increasing Cu content, enhancing light absorption. The favourable figures of dielectric constant and loss factor recorded at frequency variation makes the material suitable in high frequency performances. The conductivity response to temperature, impedance value and the semicircular nature in the Nyquist plot supports each other and justifies non-Debye type relaxation mechanism within the prepared ferrites. The Cu modified ferrites possessing a broad range of relaxation period can interact and effectively absorb an extended range of spectrum from the microwave frequency range. The combined improvements in optical, dielectric, and thermal properties confirm that Cu-modified magnesium-manganese-gadolinium ferrites are promising candidates for high-frequency, high-temperature microwave absorbing applications. The reduced dielectric loss (tanδ) at radio/microwave frequencies for certain x values makes it better for high-frequency devices.</p> Graphical Abstract <p></p>