For the last six years IBM has been selling the N series gateway and it has been a great tool to add file based protocols to traditional block storage. A gateway takes luns from the SAN storage and overlays its own operating system. One of the 'gotchas' with the gateway is the storage has to be net new, meaning it can not take an existing lun that has data and present that to another device.
Traditionally the gateway was used to put in front of older storage to refit the old technology with new features. In the case of N series, a gateway would be able to add features like snapshots, deduplication and replication. In the past few years, we have added the option to use both external and internal disk to a gateway system. The only caveat to this solution is you have to order the gateway license when the system is initially ordered. A filer can not be changed into a gateway system.
Another solution that we see in the field is when a customer is looking to purchase a new system and most of the requirement is SAN based and only a small portion is NAS. Putting a gateway in front of a XIV became a very popular solution for many years and still is today. IBM did release the SONAS platform that can be used as a NAS gateway in front of the V7000, SVC, and XIV.
I have seen some architects that wanted to use a gateway in an all NAS solution for new disks. This only complicates the solution by having to add switches and multiple operating systems.
If we look at virtualization of storage, the gold standard has been the SAN Volume Controller (SVC). This system can take new or existing luns from other storage systems and presents them as a lun to another host. This data can be moved from one storage system to another without bringing the lun offline. The IBM V7000 also has this virtualization feature as the code base for both systems are the same. The cool feature that IBM has added to the V7000 is now the system has the ability to do NAS and SAN protocols. This now competes in the same space as the EMC VNX and Netapp FAS systems.
The virtualization in the SVC code is somewhat similar to the gateway code in the N series. They both can virtualuze the lun from another storage platform. If you need to keep the data that is on the older system intact, then a SVC device is needed. I would also mention that the movement of data between storage systems is much easier with the SVC. I would also mention the N series gateway has more functionality like deduplication and easy replication than the SVC.
Finally, the SVC code was built by IBM to sit on top of complicated SAN environments. Its robust nature is complimented with an easier to use gui from the XIV platform. The N series gateway is somewhat easier to setup but is not to be used for large complicated SAN environments.
Both systems are good at what they do, and people try to compare them in the same manner. I would tell them, Yes they both virtualize storage but are used in a different manner.