The compound Cu₁₂Sb₄S₁₃, known as tetrahedrite, is an eco-friendly p-type thermoelectric material with earth-abundant, low-cost, and less toxic constituents. This contemporary work studied the thermoelectric properties of Gd and Se double-substituted tetrahedrites. The samples with nominal composition Cu_11.95Gd_0.05Sb₄S_13−xSe_x (where x = 0, 0.2, 0.4, 0.6, and 0.8) were prepared using the solid-state synthesis and sintered using high vacuum hot pressing. The structural analysis using X-ray diffraction revealed a successful tetrahedrite phase formation, and the systematic increase of lattice parameters with the selenium content (x) indicated successful Se substitution. The electron probe microanalysis revealed the presence of secondary phases Cu₃SbS₄, CuSbS₂, and CuGdS₂. The Raman spectroscopy showed a weakening Sb–S/Se bond with increased Se content in the samples. The +3 and −2 oxidation states of Gd and Se, respectively, were confirmed from the XPS study. Gd³⁺, a higher valence substituent at the Cu⁺/Cu²⁺ tetrahedral site, helped reduce the carrier concentration. On the other hand, the isoelectronic substitution of Se²⁻ for S²⁻ enhanced the thermopower and power factor by introducing resonant energy states near the Fermi level. Consequently, a maximum power factor of ∼1.35 (±0.08) mW m⁻¹ K⁻² at ∼729 K was obtained for the sample x = 0.4. The sample x = 0.4 also exhibited the lowest thermal conductivity ∼ 1.18 (±0.04) W m⁻¹ K⁻¹ at 729 K. The Callaway model indicates that the lowering in the lattice thermal part of conductivity of this sample is due to the combined effect of point defect and Umklapp scattering. The simultaneous improvement of carrier concentration and thermal conductivity resulted in a relatively high zT of ∼0.83 (±0.09) in the x = 0.4 sample.