I work at the physics department of a major university and we are developing a series of applications to interface with lab equipment, and all the development is being done is objective-c and Cocoa / GNUStep.
It is a fairly straight forward process to re-compile a Cocoa application under GNUStep, it is an excellent cross platform solution for Mac and Linux (I do not do Windows so I do not know how well GNUStep work there).
I am also working on a set of numerical quantum and fluid dynamics simulations where the visualization / user interface is using opengl with Cocoa / GNUStep and the numerics are C / lapack, and so far I have not run into any real compatibility issues between GNUStep and Cocoa.
In a previous life, I was forced to use Windows and develop in MFC and C-sharp, what a nightmare that was. It is a true joy to develop in Objective-C / Cocoa / GNUStep, far superior to any other enviornment I have worked in (well QT is not bad, but still not as nice as Cocoa).
My company is using GNUstep to develop a commercial long-range data multiplexer equipment configuration and monitoring application. It is a distributed client-server system, so yes, GNUstep can be used to do more than simple stuff. We've also written our IPTV solution in it (live streaming server, VoD server and a few other pieces of IPTV headend software).