GNUstep is a free software implementation of 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 based on and completely compatible with the OpenStep specification developed by NeXT (which has since been bought by Apple Inc.) and Sun Microsystems. Like Apple, GNUstep also has a Java interface to OpenStep, as well as Ruby 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.
GNUstep began when Paul Kunz and others at Stanford Linear Accelerator Center wanted to port HippoDraw from NEXTSTEP to another platform. Instead of rewriting HippoDraw from scratch and reusing only the application design, they decided to rewrite the NeXTSTEP object layer on which the application depended. This was the first version of libobjcX. It enabled them to port HippoDraw to Unix systems running the X Window System without changing a single line of their application source. After the OpenStep specification was released to the public in 1994, they decided to write a new objcX which would adhere to the new APIs. The software would become known as "GNUstep".