You may want to give KitKit a try, if you need to write ObjC2.0 code on linux
KitKit provides an ObjC2.0-environment using clang, cmake, gnustep-base and gnustep-libobjc2 (please note, there is not gui support yet, but this is easy to add. Checkout KitKit's install script).
In order to build and install the KitKit environment, try the following steps
(tested on a Ubuntu 12.10 x64)
git submodule update --init # this will take a while (couple of minutes)
sudo ./install # depending on your machine, this takes a lot of time since KitKit builds everything from scratch
after that, the directory /opt/kitkit-1.0-rc1 should exist.
then, update your PATH variable
export PATH=/opt/kitkit-1.0-rc1/bin:$PATH
to test if KitKit works, try to build the demo project that
comes with KitKit
cd demo/hello
mkdir build
cd build
cmake ..
then run: ./kitkit-hello
good luck :)