These are additional frameworks and libraries for GNUstep. The additional libraries which come with GNUstep always start with GS prefix, which indicate that they are not part of OpenStep specification, which start with NS prefix. You can compile GNUstep on MacOS X to get these GS classes besides Cocoa so that it is more easy to port GNUstep application to Cocoa environment. Other libraries are not in the core of GNUstep, but make GNUstep catch up the newest technology even though OpenStep specification is made in 90s.
In some case, classes in various applications can be reused under their licenses. If you need some function which can be found in other applications, search in their source code. It might save you couple time.