GNUstep is a mature Framework, suited both for advanced GUI desktop applications as well as server applications. The framework closely follows Apple's Cocoa (formerly NeXT's OpenStep) APIs but is portable to a variety of platforms and architectures.
GNUstep offers Development tools for command-line and GUI development, as well as the foundations for a Desktop environment.
Application packages are mostly available from your Linux or BSD distribution repository
GNUstep provides the foundations for a portable desktop environment, but delegates this task to other projects, like:
GNUstep provides an environment to easily develop advanced GUI desktop applications as well as server applications.
GNUstep's core framework provides open source version of the Cocoa's (formerly known as OpenStep from NeXT) APIs and tools for as many platforms as possible.
GNUstep provides a robust implementation of the AppKit and Foundation libraries as well as the development tools including the advanced interface designer Gorm (the InterfaceBuilder) and the IDE ProjectCenter (ProjectBuilder/Xcode).
Thematic instead provides a user-friendly access to theming.
GNUstep seeks to be source code compatible with Cocoa, it can
thus be used to develop and build cross-platform applications
between Macintosh (Cocoa), Unix (Solaris) and Unix-like (GNU/Linux and GNU/Hurd, NetBSD, OpenBSD, FreeBSD) platforms as well as Windows
GNUstep's main development language is Objective-C, but GNUstep is not limited to that.