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.