Abstract: Urban underground gas pipeline networks are intertwined and cross-coupled with transportation facilities, municipal utility pipelines, and commercial premises, resulting in highly complex gas leakage and dispersion. Consequently, gas explosion risks are highly concealed and uncertain, posing significant regional safety hazards. Therefore, revealing the “leakage–dispersion–accumulation” mechanism of gas within adjacent building spaces is of great significance for improving the accuracy of regional risk assessment. This study focused on the building layout associated with the Songyuan “7·4” gas explosion accident in Jilin Province. A typical adjacent-building space model was established using numerical simulation methods to systematically investigate the spatio-temporal evolution characteristics of gas dispersion under different spatial and path complexity conditions. Multiple building configurations and path connectivity models were constructed to analyze the methane dispersion behavior within adjacent spaces, with particular emphasis on the influence of architectural spatial structures on the spatio-temporal evolution of hazardous regions with methane concentrations exceeding 5%. The results indicate that, under both spatial complexity and path complexity conditions, the dimensionless growth process of the 5% methane hazardous region can be divided into three stages: slow increase, rapid accumulation, and stable diffusion. The overall evolution exhibited a characteristic “double-S curve” pattern, and methane remained in the accumulation stage for approximately 50% of the dimensionless time. Regarding spatial complexity, symmetric room layouts exhibited lower risk levels than asymmetric layouts, reducing the hazardous-region intensity by 10.09%. In terms of path complexity, the double-upward-bending path configuration presented the lowest risk, which was approximately 14.7% lower than that of the highest-risk scenario. Furthermore, under the same 5% methane hazardous region condition, the influence of path complexity on the dispersion intensity was weaker than that of spatial complexity. The evolution characteristics of methane-hazardous regions under different ventilation conditions were investigated. The results demonstrate that effective building ventilation can significantly weaken the effects of methane accumulation. When the window area accounted for more than 2% of the floor area, the area of the 5% methane hazardous region decreased by approximately 80%, and the expansion rate of the hazardous region was substantially reduced. Under the gas shutoff–ventilation coupling condition, methane concentration fluctuations were significant, and attenuation was limited under the 2% ventilation condition. In contrast, under the 10% ventilation condition, the methane concentration exhibited a short-term slow increase, followed by a continuous decline, with a cumulative reduction of approximately 43% and an average decay rate of approximately 4×10^–5%·s^–1. A comprehensive analysis indicated that before the 5% methane hazardous region expanded to 20% of the total building area, large-scale hazardous coverage had not yet occurred inside the building. Therefore, this period represents the optimal time window for personnel evacuation and emergency response. Timely implementation of evacuation, gas shutoff, and ventilation control measures can effectively reduce casualties and property losses caused by gas explosion accidents. The findings of this study can provide theoretical support and technical guidance for urban building safety design and the development of emergency evacuation strategies for gas leakage accidents.