A systematic investigation on the effects of growth temperature, Ga flux, and Sb flux on the incorporation of Sb, film structure, and optical properties of the GaN1−xSbx highly mismatched alloys (HMAs) was carried out. We found that the direct bandgap ranging from 3.4 eV to below 1.0 eV for the alloys grown at low temperature. At the growth temperature of 80 °C, GaN1−xSbx with x > 6% losses crystallinity and becomes primarily amorphous with small crystallites of 2–5 nm. Despite the range of microstructures found for GaN1−xSbx alloys with different composition, a well-developed absorption edge shifts from 3.4 eV (GaN) to close to 2 eV for samples with a small amount, less than 10% of Sb. Luminescence from dilute GaN1−xSbx alloys grown at high temperature and the bandgap energy for alloys with higher Sb content are consistent with a localized substitutional Sb level ESb at ∼1.1 eV above the valence band of GaN. The decrease in the bandgap of GaN1−xSbx HMAs is consistent with the formation of a Sb-derived band due to the anticrossing interaction of the Sb states with the valence band of GaN.