Low-dimensional materials such as gallium phosphide (GaP) and gallium nitride (GaN) nanoribbons have attracted growing interest for thermoelectric energy conversion due to their size-dependent transport properties and favorable electronic structures. In this work, we utilize a first-principles computational framework that couples density functional theory (DFT) with the nonequilibrium Green’s function (NEGF) method to systematically investigate the charge and heat transport characteristics of GaP and GaN nanoribbons. Phonon dispersion analysis reveals distinct spectral gaps—estimated at 35 meV for GaP and 50 meV for GaN—between acoustic and optical modes, which play a pivotal role in suppressing lattice thermal conductivity. Our electronic transport simulations reveal pronounced quantum confinement, leading to discretized transmission channels and energy-selective transport. The thermoelectric response, characterized by Seebeck and Peltier coefficients, exhibits sign reversals and sharp peaks near the Fermi level, reflecting intricate interactions between electron and hole contributions. Projected density of states (PDOSs) further indicates that the electronic structure is strongly influenced by edge configuration and atomic arrangement. Under optimized conditions, the figure of merit (ZT) reaches values nearing 0.5-1, suggesting strong potential for integrating GaP and GaN nanoribbons into miniaturized thermoelectric systems. These findings provide valuable theoretical insights for guiding future experimental efforts toward efficient nanoscale energy-harvesting devices.