An Oracle Database Gateway is a gateway that is designed for accessing a specific non-Oracle system. Oracle provides gateways to access several commercially available non-Oracle systems. For example, an Oracle Database Gateway for Sybase is designed to access Sybase databases.
With Oracle Database gateways, you can access data anywhere in a distributed database system without being required to know either the location of the data or how it is stored. When the results of your queries are returned to you by Oracle Database, they are presented to you as if the data stores from which they were taken all resided within a remote instance of an Oracle distributed database.