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The composition dependence of the band-gap reduction of GaAs1-xNx grown by molecular beam epitaxy and metal organic vapor phase epitaxy was investigated using transmission, reflection, and low-temperature photoluminescence (PL) spectroscopy for N incorporations ranging from doping concentrations up to x=5 × 10-2. We identified four different regimes of N incorporation with distinctly different band-gap scaling. N-doped GaAs shows sharp PL lines due to N cluster states, but no significant change in the band gap. In the ultradilute region (10-5≤x≤ 1.5 × 10-3) a strong band-gap reduction was observed which scales according to x, irrespective of the local distribution of N atoms in the As sublattice. The same band-gap scaling was observed for ultradilute InGaAsN after corrections for strain and In alloying. In an intermediate compositional region (1.5 × 10-3≤x≤2.5×10-2) ΔEg scales according to x2/3. At higher concentrations (x>2.5 × 10-2) ΔEg weakens due to effects connected with N oversaturation of the As sublattice.