We report the optical and electronic characteristics of GaP1−xBix bismide alloys with dilute composition in the cubic zinc blende crystallographic phase calculated using a scalar relativistic Full Potential Linear Augmented Plane Wave method within Density Functional Theory. The equilibrium lattice constants are calculated using the improved generalized gradient approximation of Wu and Cohen in excellent agreement with experimental values. A quasi-linear dependence of the lattice parameters on Bi composition is obtained. Electronic structures such as the band gap, total density of states and partial density of states have been discussed using the recently developed approximate modified Becke–Johnson of Tran-Blaha with spin–orbit interaction (SOI) included. Electronic band structure analysis shows a significant decrease of band gap energy when P is replaced by Bi (by ~548 meV per x = 0.062). The band gap can be tuned in the emission wavelengths range from the visible to near-infra spectrum (~0.414–1.633 μm for x ~ 0.187). The decrease in band gap energy is attributed to the both resonance interactions between Bi_p sates with the top of the valence band and Ga_s states at the host conduction band when the Bi content increased. The band gap undergoes an indirect (Γ–X) to direct (Γ–Γ) transition at x = 0.05. The complex dielectric function, refractive index, reflectivity, extinction and absorption coefficient were calculated for incident radiation energy 0–8 eV. The critical-points in various optical spectra are identified in excellent agreement with measured values. The maximum absorption occurs within the energy range from ∼5 to 8 eV, corresponding to the ultraviolet spectrum region. Dilute GaP1−xBix alloys are a promising material system for the realization of the future photonic devices operating in the near-infra spectrum.