Research on gas hydrates is relevant to many industrial issues including flow assurance in oil and gas production and transportation, exploitation of new energy deposits in the form of natural gas hydrates, gas storage, separation, and more. The thermodynamic modeling of a gas hydrate formation is a fundamental work for all gas hydrate research. Considering the drawbacks of the traditional van der Waals-Platteeuw type models, Chen and Guo proposed a new approach to the modeling of hydrate formation/dissociation based on a reaction plus adsorption two-step hydrate formation mechanism instead of the single adsorption mechanism in van der Waals-Platteeuw theory. Since the establishment of the Chen-Guo model in the 1990s, it has achieved wide applications in modeling of hydrate formation thermodynamics, kinetics, and hydrate-based technical processes. For removing the limitations of the original Chen-Guo model, a series of extensions or modifications have been done successfully by many researchers to make it suitable for different complicated systems, including structure-H and semiclathrate hydrates, sour gas/inhibitor/promoter-containing systems, water-in-oil emulsion systems, and porous systems. The aim of this review is to summarize these applications, extensions, and modifications to the Chen-Guo model as well as the two-step mechanism in recent years. Almost 190 references have been cited in this review. We hope it will be helpful for the gas hydrate community.