Having GNU Toolchain handy is always a good thing because you’ll never know when you have that urge to “download the source and compile it yourself”. However it took my a while to figure it out the first time I got my macbook air.
A lot of posts and forum threads suggest installing Xcode which come bundled with a set of toolchain, but apparently newer version of Xcode (v 4.4.1 at the time this post is written) no longer installs the binary into the standard location (/usr/bin, /usr/lib and so on) — instead it’s all placed inside /Applications/Xcode.app to fit with newer App Store style packaging. This means you can’t access the toolchain from terminal shell.
Thanks to http://stackoverflow.com/a/9420451/179630, the trick is once you’ve got Xcode installed, go to Xcode > Preferences > Downloads and install the Command Line Tools component.