We report on the evolution of the optical properties of GaP1-yNy epilayers upon hydrogen irradiation for an extended nitrogen concentration range (y = 0.05% - 1.3%). For y = 0.6%, photoluminescence measurements show that hydrogen leads to an apparent band gap widening and to an ensuing appearance of N cluster states in the forbidden gap of GaP1-yNy, Hydrogen removal from the samples results in a full recovery of the electronic properties of the as-grown material. For lower N concentration (y = 0.05%), hydrogen causes a spectral weight transfer from electronic levels associated with closer N pairs to those associated with more distant N pairs and single N atoms.