We have performed an ab-initio study of the structural, electronic, optical and thermoelectric properties of an atomically thin layer of gallium bismide (GaBi) via density functional theory. We found that the GaBi in two dimensions has hexagonal honeycomb buckled structure. The results show that the buckled GaBi monolayer has a direct bandgap of 0.036 eV at high symmetric-point. The absorption spectrum of GaBi layer has been computed from the real and imaginary part of the complex dielectric function using the Kramers-Kronig transformations. The highly intense peak in the absorption spectrum lies in the visible and ul-traviolet (UV) region of electromagnetic spectrum reveal that GaBi monolayer can be used as solar cell material and UV-detector. Furthermore, the thermoelectric properties such as Seebeck coefficient, electrical conductivity, thermal conductivity and thermo-electric figure of merit (ZT) of GaBi monolayer were also calculated using the obtained electronic band structure results within a semiclassical Boltzmann theory under a constant relaxation time approximation. The obtained high value of ZT at low temperature makes GaBi monolayer promising candidates for low-temperature thermoelectric applications. Our theoretical results will pave a way to utilized GaBi monolayer in the nanoscale electronic, optoelectronic and thermoelectric devices. [ABSTRACT FROM AUTHOR]