The NUnit team decided to make the GUI a separate product and will be releasing it separately. It is being rewritten from the ground up for NUnit 3, but hasn’t been released yet. Development is happening on the GitHub page at https://github.com/nunit/nunit-gui if you want to get involved or track the progress. Initial releases will be out soon.
Update – There have been several preview releases of the new NUnit GUI that can be found at https://github.com/nunit/nunit-gui/releases. The previews are not recommended for production use, but they work and can be used.