In this paper, we present our approach towards a modular and adaptable IoT gateway for operational data and functions in the use case of retrofitting brownfield installation for industrie 4.0. The aim is to enable the existing operational technology installation to provide accessible function endpoints that can be triggered by modern IT services. To fulfill the high demands of flexibility in the IT and the requirements towards time-critical response in the OT a highly decoupled architecture employing a streaming platform is proposed. In perspective to mediating between synchronous interaction in functional aspects and asynchronous behavior in OT-IT communication a function call gateway resolves this issue with the support of a highperformance RPC framework. For the concept to be useful for a wide range of retrofitted OT, design principles and modeling standards of Industrie 4.0 are used.