Tb_2.96-xCe_0.04Gd_xAl₅O₁₂ phosphors were synthesized through solid-state reactions. The influence of Gd³⁺ on the luminescence was investigated. Under the excitation at 460 nm, Tb_2.96Ce_0.04Al₅O₁₂ shows the characteristic emission band of Ce³⁺ with a peak wavelength at about 554 inn. After co-doping Gd³⁺ into Tb_2.96Ce_0.04Al₅O₁₂, the peak wavelength of the Ce³⁺ emission band shifts to longer wavelengths, which is induced by the increasing crystal field splitting. However, the Ce³⁺ emission intensity also decreases because the substitution of Tb³⁺ with Gd³⁺ causes lattice deformation and generates numerous structural and chemical defects. By comparing the light parameters of white light-emitting diodes (WLEDs) containing Y_2.96Ce_0.04Al₅O₁₂, Tb_2.96Ce_0.04Al₅O₁₂ and Tb_2.81Ce_0.04Gd_0.15Al₅O₁₂ phosphors, we can find that the WLED containing the Tb_2.81Ce_0.04Gd_0.15Al₅O₁₂ phosphor generates warmer light than the WLEDs containing Y_2.96Ce_0.04Al₅O₁₂ and Tb_2.96Ce_0.04Al₅O₁₂ phosphors. Moreover, the WLEDs fabricated by integrating a blue LED chip and Ce³⁺/Gd³⁺-co-doped Tb₃Al₅O₁₂ phosphors show outstanding colour stability when driven under different currents.