Much improved optical properties are obtained for the polycrystalline GaN layers grown on amorphous silica glass substrate by gas source molecular beam epitaxy using an ion removed electron-cyclotron resonance radical cell. Photoluminescence (PL) spectrum shows a strong band edge emission with a full-width at half-maximum (FWHM) as less as 60 meV at room temperature. 8K PL spectrum has only two sharp peaks at 3.49 eV (FWHM 17 meV) and at 3.44 eV (FWHM 30 meV). X-ray diffraction rocking curve indicates C-axis orientation. Atomic force microscopy image shows a strongly oriented uniform-size grain structure surface morphology. Clear correlations between the PL properties and the crystalline structures (qualities) are observed.