Apps run natively on Mac and can be built using Xcode. The minimum OS version required to run the app depends on the APIs used in the app and the Mac SDK in Xcode.
Please see developer.gnustep.org for Windows installation instructions.
The quickest way to get started with GNUstep is to try out the Gershwin desktop live ISO. Additional desktops can be found in the Gallery.
If you’d like to try an app on your existing system first, check availability in your distro’s package manager for the GNUstep libraries and apps. Otherwise, you may have to compile from the source code.
If you are interested in developing with GNUstep, see Developer.