Gas to liquid (GTL) synthetic oil, produced from natural gas, exhibits excellent environmental and electrical properties, making it suitable for use as a new type of transformer insulating oil. In order to better understand the gas generation characteristics of GTL insulating oil, this study utilizes molecular dynamics simulation to construct a microscopic system of GTL insulating oil and simulate its decomposition process under electro-thermal combined fault conditions. Through analysis of the simulation results, changes in the types and quantities of decomposition products of GTL insulating oil are examined, and the gas generation pathways and mechanisms are determined using isotope labeling. The simulation results indicate that the final products of GTL insulating oil decomposition include small-molecular gases such as C₂H₄, C₂H₂, CH₄, H₂, C₂H₆, and free radicals. With increasing fault temperatures, the decomposition of GTL insulating oil becomes more complete. Under the electro-thermal faults, temperature is identified as the dominant factor influencing decomposition, while the presence of a strong electric field further accelerates the process. Under the same fault conditions, the proportion of H₂ and CH₄ generated from GTL oil is about 5% higher than that from conventional mineral insulating oil. This study's findings can provide theoretical support and the reference for the fault diagnosis and condition assessment of transformers using GTL insulating oil.