In template: 'IGESData_IGESModel' is an incomplete type when using DownCast

Hi,
I am trying to understand why a DownCast is not working. I am starting from an IGESCAFReader and want to get the GlobalSection

I tryed this:
Handle(IGESData_IGESModel) modl = Handle(IGESData_IGESModel)::DownCast(reader.IGESModel());
and got this from the Compiler:
In template: 'IGESData_IGESModel' is an incomplete type

If I check the code in debug mode,
Handle(IGESData_IGESModel) modl = reader.IGESModel();
Shows the whole structure with the information I am looking for in the header section.
How do I get the header (GlobalSection) object?

If I try
Handle(IGESData_IGESModel) modl = reader.IGESModel();
IGESData_GlobalSection GS = modl->GlobalSection();

I get from the compiler:
In template: static_cast from 'Standard_Transient *' to 'IGESData_IGESModel *', which are not related by inheritance, is not allowed
Member access into incomplete type 'IGESData_IGESModel'

Thank you in advance