The download is made available (in Dec 2013) as a zip file, but for OSX there are is not 'installer script', so here is how I just set it up on my dev OSX laptop:
1) downloaded the binaries from http://www.gradle.org/downloads, unziped the zip and copied the gradle-1.9 folder to a local dev folder:
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2) created a .profile file in my current dev user root directory
3) edited that file to setup a GRADLE_HOME folder
4) started a new instance of Terminal, confirmed that the $PATH environment variable was updated, and executed the gradle command
References: