We have studied the low-temperature growth of gallium nitride arsenide (GaN)As layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN(1-x)As(x) alloys over a large composition range by growing the films much below the normal GaN growth temperatures with increasing the As(2) flux as well as Ga:N flux ratio. We found that alloys with high As content x>0.1 are amorphous and those with x<0.1 are crystalline. Optical absorption measurements reveal a continuous gradual decrease of band gap from similar to 3.4 to similar to 1.35 eV with increasing As content. The energy gap reaches its minimum of similar to 1.35eV at x similar to 0.6-0.7. The structural, optical and electrical properties of these crystalline/amorphous GaNAs layers were investigated. For x<0.3, the composition dependence of the band gap of the GaN(1-x)As(x) alloys follows the prediction of the band anticrossing model developed for dilute alloys. This suggests that the amorphous GaN(1-x)As(x) alloys have short-range ordering that resembles random crystalline GaN(1-x)As(x) alloys.