Make sure the Odata created in SMP 3 using integration gateway provides you the data when you test the Odata service from a web browser. Read this blog to understand how to test the Odata service created,How to use Integration Gateway with SMP 3.0 (Part 3)
What type of application you are developing ? If it is a native android app you can download source code of a sample SMP 3 android project from here, SMP 3 - Develop App and Test
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Midhun VP