If I choose to work with Couchbase lite (in a Ionic 2 app), can I make a gateway with another database in server (SQL database) using private synchronisation APIs?
The replication protocol used by Couchbase Lite (and the Couchbase Mobile stack) is related (and currently compatible with) the CouchDB replication protocol.
Going from NoSQL to SQL requires work to map between the two. I know of no current SQL database that you can sync with automatically. This wouldn't apply to just Couchbase Lite.