There is no real copy protection for GDL objects. There is however a tool, downloadable from the Graphisoft home page, which can hide the various GDL scripts for an object should it be opened in the IDE (built in editor) of ArchiCAD. There is a manual way by the XML-converter. See more here. This offers only casual protection, because it does not actually encrypt the object.
From ArchiCAD 10 onwards it became possible to create object container files (*.lcf), but AFAIK this offers no additional protection.
From ArchiCAD 16 onwards objects can get password protection to open, view and edit the source code.