XIB
As of MacOS 10.3 Apple moved away from the binary encoded nib files, and towards the xml encoded XIB files, for their interface definitions. As of 10.9 (Xcode 5) Apple changed the xib file format totally. Support of "XIB 5" files and above by Gorm is currently prepared.
Xib file format
The xib file format is actually quite easy to understand as it is a human readable xml file. For a reference see XIB File Format Definition as understod by MonoObjc.
Gorm compatibility to Xcode
As of version 1.2.24 Gorm is able to read xib and nib files from Xcode up to file version of Xcode 4 (to some extend). Gorm can save nib files, but these nib files currently cannot be read by Xcode 3 and above. This means for development of xibs/nibs you want to open using Gorm you need to use Xcode up to version 5 (Xcode 5 can save xibs in file version of Xcode 4). So you can use a Mac OS X version up to 10.10. macOS 10.11 and newer won't run Xcode 5 anymore.
Gorm did not yet follow up the changes of gnustep-gui described below.
gnustep-gui/AppKit compatibility to Cocoa/AppKit regarding xib files
As of release 0.28.0 gnustep-gui is able to interpret xib files created by a recent version of Xcode (Xcode 11 at the time writing this). Even Cocoa bindings (using KVC/KVO) will work to some extend.
See Also
Using .nib and .gorm files in parallel
You can use .nib and .gorm files in parallel to develop cross platform applications. GNUstep will load a .gorm file over a .nib file given the same name.