Sync Gateway is a component that ties together Couchbase Lite and Couchbase Server, providing key functions for securing mobile applications. It addresses data replication, authentication, and data access control, allowing users to define fine-grain security policies for read side policies at the document level and write side policies down to the field level. The Sync Function is a JavaScript function that gets executed whenever any JSON document gets written to Sync Gateway, where it can be used to manage read-write access and implement complex security rules. By configuring the Sync Function, users can control what data is visible to whom, ensuring that sensitive information remains private. Sync Gateway also supports SSL and TLS on the transport for additional data security when sending data over the wire. Additionally, file system encryption on the client-side is necessary for secure storage of local Couchbase Lite databases on mobile devices. By utilizing Sync Gateway, developers can create fully featured multi-user synchronizing experiences while maintaining control over sensitive data.