For readers of the book: Cocoa Programming for Mac OS X, 2nd ed. We have some clarifications for the chapter on GNUstep.
There are a variety of ways to help with GNUstep. The first task is probably just to get familiar with it. Download and install GNUstep, a few applications, etc. Hopefully you will like the applications so much or wish to write your own that it will be natural to want to improve GNUstep as well.
Probably the best way to get started as a developer is to contribute
additional regression tests for existing classes. You can find the
testsuite in the GNUstep subversion repository.
Each additional regression test makes GNUstep more reliable, and adding tests to the testsuite means that when you write code which uses a particular library feature, you can be confident that the feature will continue to work with later releases of the libraries because your tests will have been run before the newere reeasers were made.
Another simple way to get familiar with GNUstep is to help write documentation. A lot of documentation is written in the source code itself, and autogenerated. You could also improve on several of the manuals that explain the overall usage of GNUstep.
Here are the next steps:
Remember, if you want to contribute a reasonably large amount of code, you first need to sign a copyright assignment. This assigns the copyright to the code you write to the FSF, so that the entire GNUstep codebase can be legally defended by the FSF.