Two-Stage GITBlow, a special injection molding process developed by Polymer Processing Paderborn, combines the advantages of gas-assisted injection molding and blow molding. This is achieved by producing a preform with gas-assisted injection molding, which is then inflated into a larger cavity in the same mold. The inflation behavior is heavily influenced by the temperature distribution over the cross section of the preform. The preform is not rotationally symmetric and features functional geometries that are not inflated during the second process stage. Therefore, the temperature development during the first stage, i.e. the gas-assisted injection molding, as well as cooling and handling of the preform, is essential for the subsequent inflation. If certain thermal conditions are met, inflation and the creation of a homogeneous wall thickness is improved. The development of the necessary temperature profiles for homogeneous inflation is heavily dependent on the geometry of the gas chamber. Due to the preform's geometry, characteristic material accumulations are formed during the gas injection. These create local temperature hotspots that act as heat sources during the temperature equalization before inflation starts. Simulations were conducted to predict the gas chamber geometry and its influence on the temperature distribution and development during the process. The combination of two different simulation approaches creates a new method to analyze GITBlow and similar processes. Simulation results were verified with extensive experimental studies.