The General Internet Signaling Transport (GIST) protocol was introduced by the IETF in order to provide a generic transport protocol for signaling messages. An implementation of the GIST protocol that was developed by the Institute of Telematics was already tested in smaller testbed setups. An evaluation of GIST in large-scale scenarios can, however, only be accomplished by using a simulation framework. In this paper we describe how the existing NSIS-ka implementation was ported to the OMNeT++ simulation framework. First we provide an analysis of the differrent design principles used and then describe related challenges. Then we describe a methodology for integrating an existing real protocol implementation into OMNeT++. The feasibility of the chosen approach is naturally demonstrated by a set of evaluations.