LEC-grown GaP:Ni in which the Fermi level is pinned by either the Ni²⁺/Ni³⁺ or Ni⁺/Ni²⁺ level has been investigated by optical absorption (OA), thermally detected (TD) optical absorption, electron paramagnetic resonance (EPR) and TD-EPR. The spectroscopic features, subsequently assigned to Ni_Ga⁺, Ni_Ga²⁺ and Ni_Ga³⁺, were monitored under external illumination. The analysis of their spectral dependence together with the angular variation of a set of new TD-EPR peaks strongly suggests that the latter results are due to Ni_Ga²⁺ ions. It also provides identification of the photoinduced recharging processes involving the three charge states of isolated Ni_Ga. An orthorhombic Jahn-Teller model is presented to describe the Ni_Ga²⁺ resonances in the TD-EPR experiments.