Ion-beam-assisted molecular beam epitaxy was used to grow GaN on sapphire by reacting Ga from an effusion cell with ammonia. With the ion beam at low glancing angle and energy between 60-500 eV, periodic, nanoscale ripple and dots were observed both with and without growth. By changing the growth parameters, the dimensions could be tuned from 40 to 800 nm. The ripple wavelength was analyzed in terms of continuum models. However, the time constant for pattern formation was several orders of magnitude less than that predicted by linear or nonlinear theories. The mobile adatom concentration was calculated and found to be quantitatively reasonable, increasing with increasing net growth rate. The ripple wavelength was observed to be well developed for nitrogen ion beams, but not with Ar ions within our energy and flux range. Adding growth reduced the measured wavelength rather than increasing it.