<p>In this work, Mn⁴⁺-activated Ca₂GdTaO₆ phosphors were synthesized and the effects of Li⁺ co-doping were systematically investigated. Structural analyses confirm that Mn⁴⁺ ions preferentially substitute for Ta⁵⁺ at octahedral sites in the monoclinic double perovskite lattice. Under UV light excitation, the phosphors exhibit a narrowband red emission centered at 676&#xa0;nm, with an optimal Mn⁴⁺ concentration of ~ 0.25&#xa0;mol%. To mitigate charge imbalance and nonradiative losses induced by heterovalent Mn⁴⁺ substitution, Li⁺ ions were introduced as charge compensators. Li⁺ codoping effectively enhances emission intensity when the Mn⁴⁺ concentration is ≤ 0.25&#xa0;mol%, while no positive effect on Ca₂GdTaO₆:Mn⁴⁺ luminescence is observed at a relatively high level. The optimized CGTO:0.25Mn⁴⁺,0.25Li⁺ phosphor achieves CIE chromaticity coordinates of (0.7237, 0.2763) with a color purity of 96.9% and an internal quantum efficiency of 24%. Moreover, the phosphor exhibits moderate thermal stability, retaining over 54% of its initial emission intensity at 373&#xa0;K. A prototype phosphor-converted LED fabricated using a 365&#xa0;nm UV chip delivers a dominant far-red emission that matches well with the absorption spectrum of phytochrome P_fr, demonstrating the potential of CGTO:Mn⁴⁺,Li⁺ phosphors for indoor plant cultivation lighting.</p>