I do not have direct experience with the EMC Centera, but can make these suggestions (based on experience with EV and discussions with their training staff):
#1) Contact Symantec support for the proper guidelines surrounding the migration of this data...dealing with CAS storage solutions requires *special* handling and Symantec is usually very helpful! (EV is Centera aware, which means it likely handles the data in a different manner than if it is using NetApp)
#2) You will *likely* be performing a host based data migration, or an import/export. You cannot SnapMirror/VolCopy/etc. from EMC to NetApp.
What I am having a hard time with is the phrase "Gateway filer should be purchased, with additional disk shelves, and attach it to the Centera CAS"...V-Series Gateways are wonderful products, however, I don't think it helps you any more in this situation than a normal FAS appliance - since Centera cannot publish it's storage to the V-Series Gateway.
More than likely you will need to setup a CIFS share on the NetApp (or publish a LUN w/ NTFS...your choice), and then use a host based solution to read the data from the Centera CAS and copy it to the NetApp. Afterwards you will likely go into the EV directory database and modify the location of your archive partitions (to reflect the NetApp UNC, rather than the Celerra). Symantec support should be involved with this... they will know exactly how to get the data out of the Centera (which stores it's files quite differently than a NetApp or alternative CIFS solution) and hopefully can provide some insight on how to perform the initial baseline copy while services are still active, if possible, to avoid a lengthy outage.
Sorry I can't be of more help on the Centera side...
NetApp has solutions to migrate data from Centera to NetApp. As others have noted, the V series does not support Centera. However, there are ways to migrate designed to improve the speed of migration, as well as reduce the impact on end users.
See our PS offering here: https://tech.netapp.com/community/pstn/dpr/ndas, which will specifically meet your needs for EV migration. There are additional options for other applications, such as ECM (FileNet, Documentum, etc.) and PACS (medical imaging) which are not yet listed on the site but which are available.
Feel free to contact the Practice Director, Jag Sahni, whose contact information is listed. Alternatively, you can have your NetApp Sales rep or SE contact me directly.
Second, in the US you can contact your local PS Engagement Manager (or your NetApp sales rep can do that for you, if you are a customer) for assistance with the services. I'm still available, of course.