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GNUstep is the implementation of the OpenStep environment on Unix operating systems.
The installation of the environment is not so easy because it consists in many individual packages and libraries to install. The full procedure is well described on Dennis Leeuw's page, in the administrator section, where the principles and installation of GNUstep is well documented.
The GUI of Eidolon was not tested with GNUstep, but the project is in active development and a compatibility should be tested soon.