How to call a FM from backend

Hi

I need to understand how we from Gateway can call a FM on backend.

In backend I want to call my BAPI (a BAPI I only can find in the backend).

When I created the Datamodel, I did import the structure of my BAPI (from Backend system) to Gateway.

But in the method PURCHASEHEADER_GET_ENTITYSET of my class, I can't call a BAPI, that isn't in the Gateway system. But how do I call the BAPI then?

I have searched the system, and I can't find information that solve my problem.

http://scn.sap.com/thread/3361319

In Manage SAP System Aliases" my settings are:

SAP System Alias: ABC_400
Description: Backend

Local GW: <space>

For local App: X

RFC destination ABCCLNT400

Software Version: DEFAULT

Anyone who will help me?

Thanks

Aikon