Sync Gateway allows users to validate document types in an application by utilizing the Sync Function's implementation logic. The Sync Function is used to determine the accessibility of documents based on their type and user authentication. For example, if a document has a "friends" type, it can be processed only for the authenticated user who owns that document, while other users will not see it. Similarly, if a document has a "vehicles" or "schools" type, similar validation is applied to ensure access is granted to authorized users. The Sync Function can also be used to restrict write operations by requiring users to have access to specific channels for the documents they want to write to.