GNUstep is a free software implementation of Cocoa (formerly NeXT's OpenStep) Objective-C libraries (called frameworks), widget toolkit, and application development tools not only for Unix-like operating systems, but also for Microsoft Windows. It is part of the GNU Project.
GNUstep features a cross-platform, object-oriented development environment. Like Apple Cocoa, GNUstep also has a Java interface, as well as Ruby, Guile and Scheme bindings. The GNUstep developers track some additions to Apple's Cocoa to remain compatible. The roots of the GNUstep application interface are the same as the roots of Cocoa: NeXT and OpenStep. GNUstep predates Cocoa.