In this work, we have investigated the structural and magnetic properties of GaP-based diluted magnetic semiconductors (DMSs). Based on first-principle density functional theory (DFT) calculations and using a full potential linearized augmented plane wave (FP-LAPW) method in generalized gradient approximation (GGA), some significant structural and magnetic properties of Transition Compounds-doped Gallium Phosphide (Ga1-XTCXP: TC = V, Mn, Fe, Co, Ni amp; Cu) as DMS are investigated. Then, a conventional gallium phosphide photovoltaic junction was simulated with a GaP absorber layer as reference cell. Last, a high efficiency gallium phosphide photovoltaic junction was proposed with a Ga1-XTCXP absorber layer. Simulation results showed that by using Ga1-XTCXP compound, the short-circuit current density (J(SC)) and the conversion efficiency of proposed solar cell increase impressively. Under global AM 1.5 conditions, the proposed cell structure has an open-circuit voltage (V-OC) of 1.01 V, J(SC) of 9.05 mA/cm(2) and a fill factor (FF) of 88%; all in all lead to total area conversion efficiency (eta) improved to 8.06% which increased about 5.93% compared with a reference cell.