Sadly, it never gathered much attention outside the former NeXTStep users crowd, save for a small resurgence of interest back OS X adoption soared and a lot of people began to actually like Cocoa.
Which is quite a shame. Mail.app, for instance, was really good.
Seems Darling is still being developed as we "speak". The Github repo is showing commits as recently as 2 days ago. And it builds on Gnustep in an attempt at supporting OSX software running on top of Linux from what i can tell.
GNUStep is still being developed, much to my amazement, but somewhat slowly. Back in 2002-2003, it was actually in use for some commercial development, too, so it's not like it's a long-abandoned turd. But the development rate is fairly slow and many applications that use it haven't been updated in a while. I expect it will be all but dead in the 3-5 years it will take the lovely members of the Linux Foundation to push Wayland everywhere.
The biggest hurdles to GNUstep's adoption were a) its lack of documentation (you were generally expected to use the NeXTStep documentation, although some functions were not implemented at all and others were slightly buggy) and b) the fact that Gorm and ProjectManager were really buggy for a really long time. This made GNUStep development not significantly more pleasant than GTK or Qt development -- not to mention difficult to get into for people who had never written NeXT or OpenStep software before.
Franly if it were not for the objective-C stuff it would make for a real nice option.