Hi Duke,
the post relates primarily to my Google Summer of Code project for this year, which is a graphics backend that uses GNUstep's graphics library called "Opal" in place of libraries such as "Cairo" or "Art". The post does not cover an entire system, and if you used an existing, complete, well supported backend such as "Cairo" screenshots would also look better. The post is mostly intended for other GNUstep developers or experienced Objective-C programmers.
If you just want to start learning Objective-C, you're not interested in this backend. Hence, you can start by installing Debian or Ubuntu and installing the GNUstep-related packages they ship. Their packages are horribly, horribly out of date most of the time, so once you learn the basics, you'll want to move on to the latest releases or the latest SVN builds.
There is also "One Step to GNUstep" VM floating around - look for it in your favorite search engine.
This is the best recommendation I personally can give to a new learner. Others may have better suggestions.