Gateways between overlay virtual world and (VLAN-based) physical reality are a crucial component in every design using overlay virtual networks. Ideally one could use virtual appliances, but sometimes the users keep asking for layer-2 gateways.

The VMware NSX Layer-2 Gateways video from the VMware NSX Architecture webinar describes the use cases for layer-2 gateways and the VMware NSX implementations.