I ported Oolite from Obj-C/Cocoa to GNUstep/SDL so I could run it on Windows and Linux (me not having a Mac).

Just say no to GNUstep - it is terrible. I had to:

• Create a main function to get the game running that had to do something with memory pools and get the objects set up like the NIB did on the Mac.
• Redo all the screen handling code to use SDL rather than Cocoa (obviously you have no choice in this, even going to C++).
• Convert all plists to ASCII format because GNUstep windows can't handle XML plists. I later made a version that did but then you have to distribute the custom version. We also just added some XML plist parsing code into the game itself because most people wouldn't convert the plists in their add-ons.
• Debug using NSLog because debugging Obj-C on Windows with gdb is no fun at all - it doesn't handle Obj-C stack traces well either - lots of frames show up as ??
• Write replacements for the sound classes because GNUstep doesn't handle them as far as I can see.
  

There is probably more but I've happily forgotten most of the early porting effort.

Trying to get a Mac app running under GNUstep is a world of development pain. So is converting all your code to C++ but at least you can stop suffering once you've done it!

Note, I'm no C++ fan, but for cross-platform purposes with code already written in an OO extension to C, it is probably the best choice.

Regards,
David.