Sunil,
SAP Gateway is a OData producing tool and not designed for OData consuming.
To consume an OData service, here are the steps.
1. Create a RFC destination (type http) pointing to the OData provider.
2. Test the RFC destination connection to ensure that connection is successful. You may maintain authentications options as part of RFC destination. If it is a https destination (not http), then you will have to maintain SSL certificates in tcode STRUST.
3. In ABAP program, use this RFC destination and create a REST client using the REST client class (CL_REST_HTTP_CLIENT).
4. Use various REST methods from this class for GET, POST, PUT for calling the external OData service.
OSCI is a topic within Gateway which consumes an OData service, wraps it, and creates a new OData service out of Gateway. Its classes (especially /IWBEP/CL_OCI_DP) can give you sample code on how to consume a OData service.
Regards
Krishna