We report on the optical properties of GaN grown on 6H-SiC and (111) Si and exhibiting tensile biaxial strains varying from nearly 0 up to 4.5x10(-3). The layers have been assessed by X-ray diffractometry (strain measurements), reflectivity and photoluminescence (PL) as a function of temperature. Excitonic GaN gaps as low as 3.42 eV at 10 K are reported. For strains higher than similar to1.5x10(-3), the low temperature PL is dominated by B-related donor bound excitons, though the B free exciton is not observed in sigma geometry, as expected from selection rules. Our results also point to a similar behavior for A and B related excitonic luminescence as a function of temperature.