Good 'ol GNUStep. I think the only thing I ran that relied on it was WindowMaker, which was an excellent, minimalistic window manager back in the day.

WindowMaker and AfterStep never had any dependencies to GNUStep.

They were often used together though.

You're bringing back memories. I ran WindowMaker on my Sun desktop (Solaris 2.6, I think?), back in the late 90's. I spent days customizing that system, compiling everything from source.