If you want a proper Server REST API, you should still have a minimum of code writing to do. I suggest using Spring Data Couchbase for that. It’s right now the easiest way to create a REST CRUD API over Couchbase.
In general I would not advise using a CRUD API directly on any kind of database. I would write a proper domain API to avoid inappropriate intimacy issues.
Also, I would use the Sync Gateway only if you want offline sync.