A NAS gateway is a NAS (network-attached storage) system in which the storage media are externally attached, typically over a Fibre Channel interface. The external storage might be connected by means of a SAN (storage area network), or it might comprise a stand-alone box shared among multiple servers.
With the use of a NAS gateway, all clients "see" the file serving function as a standard NAS device. The gateway appears as a single folder on every user's computer. If more storage is added to the system, the gateway retains its single-folder interface. A principal advantage of a NAS gateway over traditional disk arrays is theoretically unlimited storage capacity (although, beyond a certain point, the performance degrades). A NAS gateway can also support iSCSI (Internet Small Computer System Interface), an IP-based standard for linking data storage facilities.
NAS gateways have proven useful for networking applications in businesses of all sizes because they can be clustered. Clustered network NAS gateways, which sit in the data path between client servers and the storage arrays they access, act as one logical server. A clustered file system (CFS) bundles the different NAS gateway servers together so that each gateway can access storage anywhere in the cluster. This configuration allows storage resources to be scaled-out and more options for planning storage capacity.
If you're considering NAS, this guide on network-attached storage devices walks you through the decision process.