While ongoing efforts and studies have been made to reduce greenhouse gas (GHG) emissions, less attention has been placed on calibrating discrepancies between calculated emissions and real-world emissions. However, since over 90% of emissions in the transportation system originate from roads, applying a precise model to calculate emissions and assess reduction policies is essential. As advanced form of calculating emissions, this study introduces correction factor application method designed to be adaptable to Korea’s official emission model, referred to as the GIR Basic Model. In the process of adaptation, correction factors were initially developed for 16 vehicle/fuel types in Korea, aligned with the U.S. vehicle/fuel classification system. Later, based on available data, these 16 categories were aggregated into three main vehicle categories: Passenger Car, Bus, and Truck. Moreover, since high variability in the length of road segments within the dataset could lead to inconsistencies with gradient values, potentially affecting the accuracy of the results, we also considered methods to minimize the length dependency in gradient calculation. Regarding the results from both link-based and grid-based gradients, we chose to use both datasets and conducted comparison scenarios. Scenarios reveal that GHG emission increased with road gradients, with grid-based gradients showing more significant emission differences compared to link-based gradients. Quantitatively, applying road gradients resulted in a 12-15% change in MAPE bewteen models, and considering gradients led to a reduction in overall emissions. Due to the frequent variations in terrain, including uphills and downhills, across Korea, it is essential to consider road gradients not only for accurate emission assessments at the local government level but also for informing policies aimed at mitigating emissions through gradient modifications.