Some people are using it
If you've got a linux box around and some time I'd say give it a shot. I think they may be on the cusp of critical mass. They even have a few GUI apps working.
Certainly its usable for non-UI stuff. The ObjectiveC core library is called foundation. There are half a dozen implementations of foundation floating around that have been used in various server apps.
The downer on the gnustep thing is their attitude. If you mention they have an issue with compatibility with an apple api they tell you to go tell apple to change.
The GSWeb stuff is being used by several companies to do projects. It looks decent.
Or you could just get a Mac :-)
What are you planning to do with it?
I think that it's extraordinarily important that we in computer science keep fun in computing. When it started out, it was an awful lot of fun. Of course, the paying customer got shafted every now and then, and after a while we began to take their complaints seriously. We began to feel as if we really were responsible for the successful, error-free perfect use of these machines. I don't think we are. I think we're responsible for stretching them, setting them off in new directions, and keeping fun in the house. I hope the field of computer science never loses its sense of fun. Above all, I hope we don't become missionaries. Don't feel as if you're Bible salesmen. The world has too many of those already. What you know about computing other people will learn. Don't feel as if the key to successful computing is only in your hands. What's in your hands, I think and hope, is intelligence: the ability to see the machine as more than when you were first led up to it, that you can make it more.
--Alan Perlis