Credits

Many people and sites have helped to gather the information condensed here. I'm not able to remember and list all of them, but this is a try:

Installation

Instruction on how to use repos from openSUSE's build service by Patrizio Bekerle
Instructions on how to compile the GNUstep frameworks with libobjc2 on Ubuntu/Debian by Patryk Laurent and a very helpful and extensive guide on compiling and installing by Andreas Fink

Fred Kiefer for numerous mails of patient help to understand and fix some bits and pieces and maintaining gnustep-gui (AppKit) of course.
David Chisnall for creating and maintaining libobjc2.
Riccardo Canalicchio and Alessandro Sangiuliano for creating the initial Rik.theme which was important to reach out for a good looking GNUstep next to the Nesedah theme from the Etoilé project (yes, that's been given up).
Bertrand Dekoninck who did great pioneering graphics work on creating an appealing GNUstep working environment and GNUstep themes.
Niels Grewe for writing DBusKit and DBusMenu - great stuff, hoping for more fixes to come!
Riccardo Mottola for maintaing numerous GNUstep apps.

Generally speaking to the GNUstep community as a whole as there are many friendly and helpful people. Thank you!

The GNUstep wiki
a small GNUstep dev tutorial
Cocoa Dev Central - Learn Objective C
Cocoa Coding - as well a very good, free tutorials on how to code Cocoa/GNUstep/Objective C (in German, sorry)

Aaron Hillegass/Adam Preble: Cocoa Programming for Mac OS X. 4th ed. Addison-Wesly, 2012.

Holger Hinzberg: Objective-C und Cocoa Praxiseinstieg. Programmierung für Mac OS X und iPhone. 2. Aufl. mitp/hjr Verlag, 2012.