ruby are assumed to be in path. Their locations can be overridden with absolute paths in the package settings.
Through Atom packages, search for rubyfmt.
Alt + ;
Cmd + ;
With a file open and identified as Ruby or Ruby on Rails, hit the above combination to apply auto format.
Format on save is enabled by default but can be disabled from the Package Settings.
If the Ruby code contains a syntax error it cannot be formatted. The syntax error will be displayed in a notification window.
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.