It is great that Informatica provides an SAP iDoc connector rather than having to manage such a complex integration manually. Connector-specific validations happen at the connector level, so it makes sense that the required GFK or GPK fields are validated as having a value at run time rather than design time.
In the version as of this writing, the validation only checks that the fields have values set, and not that the values are valid within the same iDoc. To clarify, the GFK and GPK values specific to a mapping would be impossible for the connector to validate as they are very enterprise-specific. However, while it could be assumed that validation within the specific mapping instance would occur to check if a GFK in one segment matches the GPK of the segment it must relate to, that is not the case. The developer must be careful that these values are set correctly within the mapping and valid between the segments.
© Scott S. Nelson