The GOS functionality is not available in transaction FBU3.
To achieve the GOS functionality we have to enhance the code of FBU3 and insert the following code.
This piece of code is written as explicit enhancement at the start of the subroutine cross_company_list_display in the source code of standard FM FI_DOCUMENT_CROSS_COMP_DISPLAY
Once you have written your ABAP Code as mentioned above you will get the GOS button in FBU3.
Regards,
Firoz.
Reference & Further Reading: https://scn.sap.com/docs/DOC-33485