The development of high-efficiency thermoelectric devices is a central challenge in energy-related research. Clathrate compounds provide an excellent platform for investigating the microscopic mechanisms governing thermoelectric performance owing to their unique crystal structures. In this work, we synthesized a series of Eu8GaxGe46−x samples (nominal x = 14.5–17.5) using vacuum arc melting followed by spark plasma sintering (SPS). X-ray diffraction (XRD) and magnetic measurements reveal a structural transition from the type-I (β-phase) to the type-VIII (α-phase) structure with increasing Ga content. High-temperature transport measurements conducted between 300 and 773 K show a clear semiconductor-to-metal transition as the Ga content decreases. For the first time, the high-temperature thermoelectric properties of this system are reported. A maximum thermoelectric figure of merit ZT of 0.54 is achieved at 623 K for a nominal Ga content of 16.5. To our knowledge, this ZT value is the highest reported experimentally for this material system to date. Complementary first-principles calculations demonstrate that lower Ga concentrations stabilize the β-phase, whereas higher Ga concentrations favor the α-phase. Moreover, the calculations reveal metallic behavior for the β-phase compositions with x = 14 and 15, while the α-phase composition with x = 16 is semiconducting with a band gap of 0.58 eV, in good agreement with experimental observations. The combined experimental and theoretical results elucidate the evolution of the electronic structure across the structural transition and provide valuable insight into the composition-dependent thermoelectric performance of this system.