The IETF's Next Steps in Signaling (NSIS) framework provides an up-to-date signaling protocol suite that can be used to dynamically install, maintain, and manipulate state in network nodes. In its two-layered architecture, the General Internet Signaling Transport (GIST) protocol is responsible for the transport and routing of signaling messages. The strong presence of Network Address Translation (NAT) gateways in today's Internet infrastructure causes some major challenges to signaling protocols like NSIS. The address translation mechanisms performed by common NAT gateways are primarily concerned with address information contained in the IP and transport layer headers. However, messaging associations between two signaling peers rely on address information contained in GIST data units. If a non GIST-aware NAT gateway merely adapts addresses in the IP and transport headers it will finally lead to inconsistent state installation at the signaling nodes. In this paper, we present the design, implementation, and evaluation of an application level gateway for the GIST protocol, that translates GIST messages in a way that allows for the establishment of messaging associations between any two GIST nodes across a NAT gateway.