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Capable of achieving wide control over energy band gap and following optoelectronic properties; the highly mismatched alloys (HMAs) are considered to be promising materials for solar energy conversion devices. The dramatic restructuring of energy bands and density of states in HMAs caused by the replacement of anions with distinctly-mismatched isovalent constituents could further be an important course in improving their thermoelectric efficiency. In this paper, we attempt to explore and address the composition-induced modifications in the electronic band structure and the resultant effects on optical spectra and thermoelectric coefficients of GaN 1−x Sb x based HMAs in the framework of density functional theory. We observe, the substitution of N by Sb, considerably affects its band structure and split the conduction band minimum (CBM) into sub-bands. With increasing Sb composition, the lowest sub-band stemmed from N-s electrons has experienced drastic downward shift leading to energy gap narrowing. Interestingly, the energy gap narrowing along R- Γ is found to be faster than that of Γ- Γ point leading to an amazing direct to indirect band gap crossover. On the other hand, the composition-induced energy gap narrowing stimulates the red-shift in fundamental absorption edge in both ultraviolet and the infrared regime, making the GaNSb potentially useful material for photovoltaic applications. In addition, substantial effect on the thermoelectric coefficients of GaNSb is also observed via Sb substitution. We obtain larger Seebeck coefficient, improved power factors and figure of merit (ZT) for GaNSb at low Sb substitution and found diminishing effect with the further increase of Sb composition. With enhanced Seebeck coefficient, power factor and ZT values at modest doping levels, GaNSb alloy could be a promising candidate for near or above room temperature thermoelectric applications. [ABSTRACT FROM AUTHOR]