Hi SAP mates,
Before start writing my first document i would like to thank Bince Mathew and Syam Babu for providing me such a higher insight to GPA and CREATE_DEEP_ENTITY .
Please refer the link for a ground preparation.
Entity Types - SAP NetWeaver Gateway Foundation (SAP_GWFND) - SAP Library
Since we are going to create Model for the document posted by Syam Babu.Please refer the below link before
getting your hands dirty.
STEP 1: Create a simple SAP UI5 Project
Right click on the Project Explore in the Eclipse IDE
New->Others and Type UI5
Select Application Project from the Directory SAPUI5 Application Project
Click Next so that it would navigate to another view were we will enter the Project name
Clicking on Next it would ask us to give a view name
Click Finish which would eventually creates a sample UI5 Project.
STEP 2:Create a OData Model Project
Follow the same step us above for creating a OData Project.But instead type 'OData'
Clicking on Next which would ask us to enter a model name and for the instance we will select Blank OData Model
Click on Finish so that it will open in an OData Perspective
If you havent added a OData Perspective you can easily add the same from Menu Bar
Window->Open Perspective->Others->OData
If you are unable to find the OData Perspective that means Eclipse IDE doesnt have proper plugins installed.
You can check it from Install New Software Option
If every thing is up and running lets draw something 
Below screenshot will show how an OData Perspective will look like.
STEP 3:Creating Entity Type
Drag and Drop an Entity Type From the Palette window of the Eclipse IDE.
After placing the Entity Type change the name of the EntityType to Header
Editing the name of the Entity Type can be done in Properties View in Eclipse IDE.
Likewise add other two entities to the Odata Workbench
STEP 4:Adding Properties to Entity Types
Before adding the properties to the out entities lets rename the key property.
Similar to above steps select the key property and rename it from the properties view of the OData Perspective.
Rename the Key Properties of other entities too.
Add additional properties by clicking on the highlighted button as shown in the screenshot.
Similarly add remaining properties to the entities.
STEP 5:Creating Association and Navigation
For this Data Model the Association is established between
1)Header and ItemO
2)Header and ItemT
So the Uni Directional Association should be established between above mentioned entities.
Click the Uni Directional Association button as highlighted in the below screenshot and connect it to the required entity.
Edit and rename the Navigation Properties that has been created by default.
Basic Design has been completed.So save the model and lets export the Model to a XML file so that we can import
to out Gateway Server.
STEP 6:Exporting an OData Model
Right Click on the Project Directory in the Project Explorer
Export->Export
Type OData Model to select export Destination.
Clicking the Next Button so that we can browse the model as shown in the below screenshot.
Provide a Export Path were we will export the model in XML format.
Click Finish.Then you are done.
You can learn about importing the Model from my next Document From OData Modelling To Service Execution using GPA:PART-2
Regards,
Vishnu