Document based applications
Document based applications are those that are primarily concerned with user documents, allowing multiple (context-sensitive) document windows and a consistent mechanism for file loading and saving. GNUstep provides a model and associated services, for the rapid development of applications following this paradigm.
Document based applications in GNUstep use NSDocument and NSDocumentController to support and coordinate this behaviour. Below are some articles describing how you can add this functionality to your program, and the role these classes in play in gnustep-gui.
Creating document based applications
Sašo Kiselkov has put together several video tutorials, one of which shows the process of creating a document-based application using ProjectManager.