Nitrogen substitution and formation of point defects in GaP(1-x)Nx layers (x ranging from 0.01 to 0.04) grown on GaP substrates are characterized by channelling Rutherford backscattering, nuclear reaction analysis and positron annihilation spectroscopy measurements. It is observed that the substitutionality of nitrogen into GaP decreases from a value of 0.91 to that of <0.1 with increasing nitrogen content from x = 1.7% to x = 4.0%. In addition to substitutional nitrogen atoms, GaPN layers have nitrogen interstitials, nitrogen clusters and defect complexes composed of multiple nitrogen atoms. Positron annihilation spectroscopy of GaPN layer shows positron trapping not only in vacancies but also trapping due to nitrogen clusters. In addition, the footprint of different nitrogen cluster states and point defects is observed in temperature dependent photoluminescence measurements.