In response to the European Union’s ambitious climate targets for 2030, green hydrogen has emerged as a means of decarbonizing energy systems and enhancing energy security. The current thesis studies the potential integration of the Greek electricity and natural gas systems through the injection of green hydrogen produced via electrolysis from surplus renewable energy. Greece’s rapidly rising RES installed capacity has led to growing levels of curtailment, which jeopardizes system efficiency and undermines investment incentives. Exploiting this surplus energy for hydrogen production offers a pathway to mitigate curtailments while accelerating the energy transition. The study combines an estimation of Greece’s 2024 renewable energy surpluses, based on published data, with a hydraulic simulation model of the NNGTS. The model, implemented in Python, simulates steady-state gas flows and examines the system’s capacity to accommodate hydrogen injections via the planned hydrogen-ready infrastructure in West Macedonia, by exploring the technical boundaries of hydrogen integration within the gas transmission network. Estimation results show that Greece experienced approximately 898.5 GWh of energy surplus in 2024, primarily concentrated in spring and autumn. After accounting for electrolyser efficiency, about 526.5 TWh of green hydrogen energy could be feasibly injected into the gas network—utilizing 58.6% of the total available surplus. The findings indicate that while hydrogen blending into the NNGTS is technically viable and effective in absorbing a significant portion of curtailed energy, it is limited by infrastructural and compositional constraints. This work adds to the discussion on integrating the electricity and gas sectors and shows how green hydrogen can help connect the two in Greece’s transition to a cleaner energy system. Future studies should focus on how the system performs under real-time conditions and on the financial viability of the model proposed.