Climate-Neutral Smart Cities and Their Energy Hinterland: Renewable Gas Integration Strategies for Romanian EU Mission Cities

European smart cities face a distinct decarbonisation challenge in the gas component of urban energy consumption, which cannot be addressed through internal optimisation alone. The European Union (EU) Mission for Climate-Neutral and Smart Cities selected 112 cities for climate neutrality by 2030, including Bucharest Sector 2, Cluj-Napoca, and Suceava in Romania. The Mission timeline coincides with the Russian pipeline gas phase-out under Regulation (EU) 2026/261, the 5 August 2026 transposition deadline for the Hydrogen and Decarbonised Gas Market Package (also referred to as the Fourth Gas Package), and the post-Hormuz energy market environment of 2026. This paper develops a hinterland framework that links Romanian Mission cities to their rural renewable gas supply through biomethane and renewable hydrogen production, integrated with circular bioeconomy strategies and routed through the national transmission system operated by Transgaz. The empirical analysis combines documentary evidence from the May 2026 consultation on Government Emergency Ordinance 9/2026, institutional documentation of Transgaz and Romgaz, and the comparative European literature on smart city renewable energy integration. Findings show that Mission city decarbonisation pathways depend on rural-to-urban gas infrastructure calibration under the post-2024 regulatory cascade. The paper proposes specific deployment strategies for each Romanian Mission city.