gem-open opens the source directory of the given gem.
To open the directory of the latest version of the given gem.
$ gem open rake
To open the directory of a specific version.
$ gem open rake -v 0.8.7
To open the directory by Mac OS X Finder.
$ GEM_EDITOR=open gem open rake
The program used to open the directory is taken from first found one from the environment variables GEM_EDITOR, VISUAL or EDITOR.