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Green gas is a promising renewable energy carrier compatible with existing gas networks, whose environmental impact and capacity to decarbonise the energy sector is evaluated by life cycle assessment (LCA). This articles reviews 42 LCAs applied to biomethane, produced by anaerobic digestion, and bio-SNG, produced by gasification and methanation, discussing the main methodological choices and their effects on the results, and highlighting the limits of the use of LCA as a stand-alone approach in real-case applications. While uncertainty analysis was performed in 34 of the reviewed studies, only 3 studies integrated the LCA with process modelling or geospatial modelling. The lack of data for pre-commercial or newly-commercialised technologies has necessitated to the introduction of thermodynamic models giving mass and energy flows, especially in the case of bio-SNG. Limits due to geospatial case-specific constraints have been overcome by two studies introducing geographical information systems (GIS) based models to evaluate the impact of green gas production system on a regional level. Facility siting and sizing has been also found to be fundamentally important in evaluating the trade-off between profitability and environmental impact. Finally, this work highlights the need for a hybrid LCA, in which LCA is integrated with thermodynamic models of the process, GIS-based infrastructure design, and uncertainty quantification, in order to inform stakeholders of the economic, environmental and energy potential of green gas production systems.