How to install GNU Objective-C on OS X?
The most straightforward way to install an Objective-C compiler is by installing Xcode (which is available free from the App Store). It gives you both the IDE and the command-line compilers (there's an option inside Xcode to install the latter). If you don't want to install the IDE, you can install the command-line tools only using xcode-select --install. 

Once you have the command-line tools installed, you can use them to bootstrap GCC if you'd prefer to use that.