GNUstep CoreData Model File Format The default extension to use for archived contents of NSManagedObjectModel in the GNUstep Core Data framework should be "gsdatamodel". The format can be found in NSManagedObjectModel.m, namely in the implementation of the methods [NSManagedObjectModel-encodeWithCoder:] and [NSManagedObjectModel-initWithCoder:]. By default, you should use keyed archives (as that is what the methods for explicit loading of data models from files expect).