Since its introduction, the Internet of Things (IoT) has changed several aspects of our lives, leading to the commercialization of different heterogeneous devices. In order to bridge the gap among these heterogeneous devices, in some of the most common IoT use cases-e.g., smart home, smart buildings, etc.- the presence of a gateway as an enabler of interoperability is required. In this paper, we introduce the concept of a Gateway-as-a-Service (GaaS), a lightweight device that can be shared between different users thanks to the use of virtualization techniques. Performance has been evaluated on real hardware and results demonstrate the lightweight characteristics of the proposal.