Virtualization enables the sharing of a same Wireless Sensor Network (WSN) by multiple applications. However, in heterogeneous environments, Virtualized Wireless Sensor Networks (VWSNs) raise new challenges such as the need for on-the-fly, dynamic, elastic and scalable provisioning of gateways. Network Functions Virtualization (NFV) is an emerging paradigm that can certainly aid in tackling these new challenges. It leverages standard virtualization technology to consolidate special-purpose network elements on top of commodity hardware. This paper presents a case study on NFV based gateways for VWSNs.