Hello Nandish,
Just to remind you that you need to fire your GET operation with $expand key word by passing the navigation property defined between your Header and Item entities.
The response of this should be used as the payload.
When you do this way you will get these kind of errors.
So please check.
Property names are Case Sensitive.
You need to make sure that you pass same Property Names which are mentioned in your Gateway Model.
Please check the property names and pass them in payload accordingly.
Regards,
Ashwin