The primary aim of this research study is to explore the optoelectronic properties of GaPBi and use these properties to design a PIN photodetector (PD) using the COMSOL Multiphysics platform. These optoelectronic properties were computed using a simulation technique based on density functional theory (DFT). Consequently, the incorporation of 3.1% Bi into GaP reduces the bandgap by approximately 439.5 meV. Furthermore, the PIN PD exhibits high photocurrent, low dark current, and maximum responsivity. Nevertheless, considering these properties, the proposed design of the PIN PD exhibits adequate performance for the detection of mid-infrared (MIR) wavelengths.