Velu,
Most of this depends on your developer, I would say if you are doing a simple app that can Create, Read, Update, Query (say an equipment), AND you were writing it all custom, it shouldn't take them more than a 15 days to develop, as for the Gateway side it should only take you 1 or 2 days to setup and configure the service.
To build a simple UI5 app that lets your Create, Read, Update, Query, again depends on your developer but I would think 10 days should be enough.
I am not sure why you think the Gateway work would take so long?
-Mike