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.  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.