This has been a big question lurking at the back of my head. From what I see, GnuSTEP nowadays is primarily a "hobby" project of a small but tightly-knit group of people. However I haven't seen a large commercial (off-the-shelf) application that uses it, apart from the small applications that comes with the GnuSTEP distribution.
Heck, since even Ubuntu doesn't really use it then is GnuSTEP really more than being a "hobby" framework?
I know that Sony's SNAP at one brief moment uses GnuSTEP, but they killed the platform before it can do anything meaningful.