The stability, structural, magnetic, and band structure properties of Y₃Fe₅O₁₂, Y_2.75Bi_0.25Fe₅O₁₂, Y₃Fe₄GaO₁₂, and Y_2.75Bi_0.25Fe₄GaO₁₂ solid solutions were studied by first-principles calculations. The results demonstrated that the substitution of Bi³⁺ induces a reduction in the bandgap of Y₃Fe₅O₁₂. At the same time, Ga³⁺ was introduced in both the octahedral and tetrahedral sites of Fe³⁺ and it was found that the bandgap remained almost unchanged. On the other hand, the magnetization was reduced with the incorporation of Bi³⁺, coinciding with the experimental trend observed in the literature while increasing with Ga³⁺ incorporation. This behavior is attributed to the disruption of the magnetic sublattices. These results help to understand the ferrimagnetic behavior of YIG and encourage its use as a promising material for optoelectronic and spintronic applications. It is also shown that there is a margin for manipulating the magnetic response through appropriate doping.