The role of arsenic in gas-source molecular beam epitaxy of GaN(As) was investigated. We found that arsenic mainly acts as a surfactant which dramatically improves the surface morphology of GaN when the growth temperature is over 700 degrees C, while it is incorporated into GaN as high as 1% at lower growth temperature. The optical absorption of the sample GaNo0.99As0.01 shows a direct-band gap optical absorption edge, but smaller band gap compared to that of GaN.