In this work, we study the nitrogen-phosphorus competition during the molecular beam epitaxial growth of GaPN alloy with N content ranging from 0 to 6%. N2 decomposition in the valved RF plasma cell is first optimized through the spectroscopic analysis of the plasma luminescence. Strain relaxation process and determination of N content in GaPN layers are then clarified using a reciprocal space mapping around the (224) Bragg reflection. Influence of growth temperature and phosphorus beam equivalent pressure (BEP) on the nitrogen incorporation is finally studied. It is demonstrated that nitrogen incorporation is mainly governed by temperature and phosphorus BEP, through the nitrogen-phosphorus competition. A good control of the nitrogen content is achieved when the temperature is low enough to avoid irreproducibility due to N or N2 desorption, and the phosphorus BEP high enough to limit the effects of nitrogen-phosphorus competition. Finally, a good accuracy on the nitrogen content control is achieved for a wide range of nitrogen composition, using effects of growth rate and valve opening.