GNUstep is an open-source implementation of the Cocoa framework, which is used for developing applications on macOS. It provides a set of libraries and tools that allow developers to create software that can run on various operating systems, including Linux and Windows. The project aims to provide a free alternative to proprietary software, enabling users to build applications with a similar look and feel to those found on Apple platforms. GNUstep supports the Objective-C programming language and offers a rich set of features for graphical user interface design and application development.