The identification and accurate quantification of greenhouse gas (GHG) have become a key challenge for scientists and policymakers working on climate change. Under the conceptual framework of the IPCC for the net GHG flux of reservoir, the International Hydropower Association analyzed the 223 reservoirs with CO₂ and CH₄ emissions data from the actual and past scientific literature to develop the G-res Tool, whether for an existing reservoir or planned reservoir project can provide an estimate of the net GHG flux. This paper introduces the basic principle and model framework of the G-res Tool model. We conducted the preliminary analysis of 12 typical reservoirs in the upper reaches of the Yangtze River in China, which were involved in the model built-in database. The average net flux of greenhouse gas in the reservoirs was 88.17 g CO₂e/(m²·a), which ranked 11.67% and was in the low threshold range among about 7000 reservoirs globally. Comparing the evaluation results of each part in the model, the contribution of emissions from the reservoir due to Unrelated Anthropogenic Sources (UAS) in the post-impoundment GHG balance from the catchment (Post) was much higher than in the pre-impoundment GHG balance from the catchment after introduction of a reservoir (Pre). Based on the post-impoundment GHG balance from the reservoirs in the upper reaches of the Yangtze River in China, it was estimated that CH₄ and CO₂ fluxes contributed quite to the greenhouse effect. After analysis and comparison of the published GHG flux data of the 12 reservoirs involved, G-res Tool was easy to operate and showed a wider application range. However, G-res Tool, as an empirical model, still has few internal defects in its basic principles and model design parts, which may cause some uncertainties and limit its application range. For case reservoirs, long-term follow-up observation and mechanism study are still the main methods to reduce the uncertainty of the net GHG flux assessment in the future.