ruby is assumed to be in path and
rubyfmt.rb is assumed to be in
~/bin/rubyfmt.rb but this can be overriden 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 disabled by default but can be enabled 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.