A gas turbine is usually installed inside a package to reduce the acoustics emissions and protect against adverse environmental conditions. An enclosure ventilation system is keeps temperatures under acceptable limits and dilutes any potentially explosive accumulation of gas due to unexpected leakages. The functional and structural integrity as well as certification needs of the instrumentation and auxiliary systems in the package require that temperatures do not exceed a given threshold. Moreover, accidental fuel gas leakages inside the package must be studied in detail for safety purposes as required by ISO21789. CFD is routinely used in BHGE (Baker Hughes, a GE Company) to assist in the design and verification of the complete enclosure and ventilation system. This may require multiple CFD runs of very complex domains and flow fields in several operating conditions, with a large computational effort. Modeling assumptions and simulation set-up in terms of turbulence and thermal models, and the steady or unsteady nature of the simulations must be carefully assessed. In order to find a good compromise between accuracy and computational effort the present work focuses on the analysis of three different approaches, RANS, URANS and Hybrid-LES. The different computational approaches are first applied to an isothermal scaled-down model for validation purposes where it was possible to determine the impact of the large-scale flow unsteadiness and compare with measurements. Then, the analysis proceeds to a full-scale real aero-derivative gas turbine package. in which the aero and thermal field were investigated by a set of URANS and Hybrid-LES that includes the heat released by the engine. The different approaches are compared by analyzing flow and temperature fields. Finally, an accidental gas leak and the subsequent gas diffusion and/or accumulation inside the package are studied and compared. The outcome of this work highlights how the most suitable approach to be followed for industrial purposes depends on the goal of the CFD study and on the specific scenario, such as NPI Program or RQS Project.