Coping with the complexity of future energy grids and the rising challenges of the energy transition to more renewable energy sources (RES), an Energy Hub Gas (EHG) concept appears to be a promising approach. This concept combines various technical components to a sector-coupling system network to support the electricity grid with ancillary and balancing services to cope with the fluctuating generation by RES and to provide (renewable) energy carriers. Additionally, the EHG serves as regional gateway and as a converter for large, centralized RES-feed-in and aggregation/distribution hub of local RES-feed-in. For combining several separate models from different domains to an EHG system model, a co-simulation approach is used with high regard on flexibility concerning the modelling aspects as well as high modularity to easily adapt the concept to further use cases. As main results presented in the paper, the coherence of the extended EHG system model and its usability for implementation in co-simulation can be shown in first simulations.