In this paper, we propose a new hydrodynamic model for gas-liquid two-phase flows in undulant pipelines based on the gas tracking method. The motion of three main forms of gas phase, including elongated bubbles, small bubbles, and gas pockets, is modeled by respective momentum equations. The mass transfer behaviors among the three gas forms, including interior mass transfer of single gas form, are considered. Therefore, the flow patterns along the pipeline can be predicted based on the gas tracking method. Afterward, the integrated pressure gradient of the gas-liquid two-phase flow in undulant pipelines can be obtained with the updated flow patterns. The model validation using the field data of a real pipeline in China shows that the accuracy of this model is within acceptable range. According to the simulation results, how the pipe terrain affects the gas transport and flow patterns is revealed, and sensitive analysis is carried out. These findings are of great practical value to obtain a deeper understanding of gas-liquid two-phase flows in undulant pipelines.